Are certain file types, like .txt or images, safer to attach than PDFs?

While text files (.txt) are generally considered safer, most email security systems can handle common image formats (.jpg, .png, .gif) and PDFs without automatic rejection, provided they are clean and from a reputable sender. However, the risk of triggering spam filters or being flagged for security concerns is significantly higher with any attachment compared to text-only emails or emails with links.

If I link to a PDF instead of attaching it, can it still affect deliverability?

Yes, even when you link to a PDF, the email server may still scan the linked content. If the hosted PDF contains malicious code, questionable external links, or is unusually large, it could still negatively impact your email's deliverability. Always ensure your linked PDFs are clean, optimized, and hosted on a secure, reputable server.

Does the file size of a PDF attachment affect email deliverability?

Yes, attachments increase the overall size of your email. Large email sizes can slow down delivery, potentially lead to timeouts, and exceed the size limits of some email providers. This can result in your emails being bounced or sent to spam, thus hurting your deliverability. Most email service providers recommend keeping total email size, including attachments, under 10MB.

Why are PDFs specifically viewed with suspicion by spam filters?

The primary reason is security. PDFs have been historically exploited to carry malware, viruses, and phishing attempts. Email providers, in their effort to protect users, implement strict filtering rules for attachments. Even legitimate PDFs can get caught in these filters, especially if they are unexpected or come from senders with a low reputation.

Is it ever a good idea to attach a PDF to a marketing email?

While it can be tempting to include PDFs for rich content, it's generally not recommended for marketing emails. The risks to deliverability and sender reputation are high. Instead, consider creating a dedicated landing page on your website with the content or incorporating key information directly into the email body, linking only when absolutely necessary for supplemental materials.

Does attaching a PDF to an email hurt deliverability? - Content - Email deliverability - Knowledge base

The question of whether attaching a PDF to an email hurts deliverability is a common one, and the answer is nuanced. While it's not always a definitive yes, it significantly increases the risk of your emails landing in spam folders or being blocked outright. Email service providers (ESPs) and internet service providers (ISPs) view attachments, especially PDFs, with a higher degree of suspicion due to security concerns and their potential impact on server resources.

This isn't just about file size, though that plays a role. It's primarily about security, sender reputation, and recipient behavior. Modern email security systems are designed to detect and quarantine potential threats, and attachments are a common vector for malware, viruses, and phishing attempts. Even legitimate attachments can inadvertently trigger these sophisticated filters, leading to deliverability issues.

Risks of email attachments for deliverability

Attaching PDFs directly to emails can pose significant challenges for email deliverability. One major reason is the heightened scrutiny from spam filters and email security gateways. These systems are constantly scanning for malicious content, and PDFs, unfortunately, have historically been used as carriers for viruses or malware. Even a perfectly safe PDF can be flagged if the filter's heuristics are too aggressive or if it detects unusual elements within the file structure.

Another factor is file size. Large attachments consume more bandwidth and server resources, which can slow down email delivery and potentially lead to messages timing out or being rejected. While file size limits vary by provider, exceeding a certain threshold (often around 10MB total email size, including attachments) can negatively affect your chances of inbox placement. Many email marketing tools, in fact, restrict or discourage the use of attachments for this very reason.

Furthermore, recipient behavior plays a crucial role. Unexpected attachments can raise red flags for recipients, causing them to mark your email as spam or simply delete it without opening. This negative engagement directly harms your sender reputation. A low sender reputation can lead to future emails being routed to the spam folder, even those without attachments, damaging your overall email program.

Technical and security implications

When an email contains an attachment, it undergoes a more thorough scanning process by the recipient's email server. This includes checks for known malicious signatures, unusual file structures, and embedded scripts. The version of the PDF specification used can also influence how easily an inspection engine can understand and trust the file. Older, simpler PDF specifications (like v1.4 or v1.6) are often preferred because they have a lower attack surface, meaning they support fewer complex features that could potentially be exploited.

Beyond the attachment itself, the presence of specific content within a PDF, even a linked one, can still trigger security concerns. Email servers will often follow links within emails to assess the content, including links to PDFs. If the linked PDF contains external links, embedded media, or other suspicious elements, it can still negatively impact deliverability, even if the PDF isn't directly attached. It still needs to pass the smell test of desktop antivirus software and email server scans.

PDF best practices for deliverability

Version: Use older, simpler PDF specifications like v1.4 or v1.6 for broader compatibility and lower security risk.
Content: Avoid embedding external links, rich media, or executable content directly within the PDF if possible.
Size: Optimize PDF file size to keep it as small as possible, even when hosting externally.
Purpose: Only use PDFs when necessary, such as for digital signatures, print-accurate documents, or invoices.

Sender reputation and user experience

Your sender reputation is a critical component of email deliverability. Every action taken by your recipients affects it, including how they interact with your emails, or if they mark them as spam. When emails with attachments are frequently flagged as suspicious or go unopened, it signals to ISPs that your sending practices might be problematic. This can lead to your domain or IP address being placed on a blocklist (or blacklist), severely impacting future deliverability.

User experience is equally important. Recipients today are accustomed to concise, easily digestible email content. An unexpected attachment, especially a large one, can be inconvenient to download, view, or even open on mobile devices. This friction can lead to a poor user experience, increasing the likelihood of complaints or disengagement. Even for transactional emails like invoices or tickets, it's often better to embed essential information within the email or provide a secure link to a hosted document, rather than attaching the file directly.

Consider the context of your email. If it's a cold outreach or a marketing email, an attachment is almost always a red flag. For established relationships, like sending a client a contract or an insurance document, attachments might be more acceptable because the recipient expects them. However, for general marketing or informational purposes, the risks associated with attachments generally outweigh the benefits, making them a significant deliverability pitfall.

Attaching PDFs

When you attach a PDF, it becomes part of the email's payload. This increases the overall size of the email, making it slower to send and more likely to exceed size limits imposed by mail servers. It also means the recipient has to download the file before viewing it.

Attachments can trigger aggressive spam filters due to security concerns like malware or phishing. Even if your PDF is legitimate, the mere presence of an attachment can lead to your email being routed to the spam folder or blocked by the recipient's email system. This directly impacts your sender reputation.

Linking to PDFs

By linking to a PDF hosted on your website or a cloud storage service, the email itself remains lightweight. Recipients can then click the link to view or download the document at their convenience. This approach improves loading times and reduces the chances of hitting size-based blocks.

Linking generally bypasses attachment-specific spam filter rules, as the email content is cleaner. While the linked content itself can still be scanned for malicious activity by some email servers, the initial hurdle of the attachment is removed. This can lead to better inbox placement and improved trust from ISPs, positively influencing your domain reputation.

Better alternatives to attaching PDFs

Given the deliverability risks, I always recommend exploring alternatives to direct PDF attachments. The most straightforward solution is to host the PDF on a web server or a cloud storage platform (like Google Drive, Dropbox, or your own website's media library) and simply include a link to the document within your email. This keeps your email lightweight and helps avoid attachment-related spam triggers.

For content that doesn't strictly require a PDF format, consider converting it into a web page. This allows for better responsiveness across devices, easier updates, and the ability to track engagement (e.g., clicks on sections, time spent on page). If the content is simple instructions or marketing material, it often makes more sense to integrate it directly into the email body using HTML/CSS or create a dedicated landing page. This offers a seamless user experience and avoids forcing recipients to download a file they might not want.

Many email deliverability experts, and even email marketing platforms like Mailjet, advocate for rethinking PDFs in your emails due to the various issues attachments can cause. While linking to a PDF still requires the file to pass security scans, it's a much safer approach for your email's journey to the inbox. It also puts the control in the recipient's hands, allowing them to choose whether to access the content. The impact on email deliverability is significantly reduced compared to attaching the file directly.

Example of linking to a PDF in an emailHTML

<p>You can find our latest brochure here: <a href="https://www.yourwebsite.com/brochure.pdf">Download our brochure</a></p>

Ultimately, if your primary goal is to get your message into the recipient's inbox and maintain a strong sender reputation, minimizing the use of direct attachments and opting for linked content or web pages is the superior strategy. It aligns with modern email best practices and enhances the overall experience for your audience.

Views from the trenches

Best practices

Always host PDFs on a reliable web server or cloud storage and link to them in your emails.

Convert instructional content into HTML within the email or on a dedicated landing page.

Keep linked PDF files optimized for size and use simpler PDF versions (1.4/1.6) for compatibility.

Inform recipients beforehand if you intend to send an email with an attachment to set expectations.

Common pitfalls

Sending large PDF attachments that slow down delivery or exceed recipient server limits.

Attaching PDFs for marketing or general information that could easily be web content.

Including unexpected PDF attachments, leading recipients to mark emails as spam.

Using complex PDF features or embedded content that can trigger aggressive spam filters.

Expert tips

If it's the same PDF for everyone, hosting it online and linking is always the more efficient and safer approach.

PDFs are generally only necessary for documents requiring digital signatures, precise print layouts, or specific legal/transactional purposes.

Even linked PDFs need to pass antivirus and security scans on the server side, so keep their content clean.

For content like product setup instructions, putting it directly in the email body or on a dedicated landing page often serves the user better than a PDF.

Expert view

EmailKarma from Email Geeks says that while the reference might be old, the principles of avoiding direct attachments for deliverability remain relevant.

2017-05-24 - Email Geeks

Marketer view

A marketer from Email Geeks says that security training emphasizes not accepting PDF attachments from external organizations, highlighting the inherent distrust associated with them.

2020-04-07 - Email Geeks

Final thoughts on PDF attachments

While transactional emails with expected PDF attachments (like invoices or tickets) might navigate filters successfully, general marketing or cold emails with PDFs face a much tougher battle. The risks of triggering spam filters, exceeding size limits, damaging your sender reputation, and providing a poor user experience generally outweigh any perceived benefits. Therefore, attaching a PDF to an email often does hurt deliverability, making it a practice to avoid for most email campaigns.

The best approach is almost always to host your PDF externally and provide a clear link within your email. This ensures your email remains lightweight, less suspicious to security systems, and more user-friendly. By prioritizing deliverability and user experience, you significantly increase the chances of your messages reaching the inbox and achieving their intended purpose.