Do PDF attachments negatively impact email deliverability and what are the best practices?
Michael Ko
Co-founder & CEO, Suped
Published 28 Jun 2025
Updated 17 Aug 2025
5 min read
The convenience of attaching a PDF directly to an email can be tempting, especially when sharing reports, invoices, or other important documents. It seems like a straightforward way to provide information without extra steps.
However, this seemingly simple act often comes with hidden risks that can significantly impact your email deliverability. Internet service providers (ISPs) and email clients are increasingly cautious about attachments due to security concerns, often flagging them as suspicious.
Understanding these potential pitfalls and adopting best practices is crucial to ensure your emails reliably reach their intended inboxes, maintaining a healthy sender reputation.
The real impact of PDF attachments on deliverability
Attaching PDFs to your emails can indeed have a negative impact on email deliverability. While it might seem like a minor detail, it sends specific signals to spam filters and mailbox providers that can hinder your message from reaching the inbox. This is a common issue that many email senders face, often leading to emails being diverted to spam folders.
One primary concern is the potential for malware. PDFs, like other file types, can be exploited to carry viruses or malicious code. Because of this inherent risk, email security systems are designed to be highly suspicious of attachments, especially from unknown or less reputable senders. This caution means your legitimate email could be caught in the crossfire.
Beyond security, file size also plays a significant role. Large attachments increase the overall size of your email, making it slower to transmit and more resource-intensive for both your sending infrastructure and the recipient's mail server. This can lead to increased bounce rates, especially if recipients have mailbox storage limits, and can negatively affect your sender reputation over time.
The risk of attachments
Email marketing tools generally advise against attaching files directly to mass emails. This is because attachments, particularly PDFs, are known to trigger spam traps and other spam filters, significantly harming your email deliverability. Even legitimate businesses can find their emails blocked or sent to junk folders due to these attachment-related red flags.
It's worth noting that some experts suggest only plain text files (.txt) are safe to attach, as other formats like PDFs can be viewed as malicious by anti-spam filters. This underscores the general sentiment to avoid attachments whenever possible.
Why direct attachments are often problematic
Beyond deliverability issues, directly attaching PDFs can also create poor user experiences. Recipients might find their inboxes cluttered with large files, especially if they receive multiple versions of a living document that gets updated frequently. This can lead to confusion and difficulty in tracking the latest information.
From a sender's perspective, attachments offer no real-time engagement data. You can't track whether a recipient has opened or viewed the attached PDF, unlike when you link to content hosted on your website. This lack of metrics makes it challenging to gauge interest, optimize your content strategy, and assess the effectiveness of your campaigns.
Furthermore, sharing sensitive information via email attachments can pose security risks if not handled correctly. While PDFs themselves can be secure, the method of transmission and storage within recipient inboxes might not always be. Hosting documents on a secure server and providing a link offers greater control and auditability.
Best practices for sharing documents via email
Instead of attaching PDFs, the best practice is almost always to host the document on your website or a cloud storage service and then provide a direct link within your email. This approach addresses the deliverability concerns, improves the user experience, and gives you valuable insights.
When you link to a PDF, the email itself remains lightweight, reducing the chance of being flagged by spam filters due to size or perceived security threats. It also allows you to track click-through rates, which is a key engagement metric for your email campaigns.
For transactional emails, where the content of a PDF might be legally or functionally critical, a link remains the preferred method. You can still ensure the recipient gets the document they need while maintaining optimal email deliverability. If you must use attachments for specific scenarios, ensure the file size is minimal, ideally less than 10MB, though even smaller is better.
Tips for linking to PDFs
Host securely: Upload your PDFs to a trusted server or cloud storage. This ensures the document is accessible and secure.
Track engagement: Utilize link tracking to monitor how many recipients click on your PDF links, providing valuable insights.
Consider alternatives: For some content, a dedicated landing page or an HTML version of the document might be more effective and mobile-friendly than a PDF. You can explore alternatives to PDF attachments.
Views from the trenches
Best practices
Always link to external documents rather than attaching them directly to your emails.
Prioritize a good user experience by not cluttering inboxes with large attachments.
Communicate clearly in your email what a link will lead to, especially if it's a PDF.
Use secure hosting for your documents to protect sensitive information.
Common pitfalls
Sending bulk emails with large PDF attachments can negatively impact your sender reputation.
Failing to track engagement when sending documents via email means missing valuable insights.
Using attachments for living documents creates confusion with multiple outdated versions.
Ignoring the carbon footprint of sending large files to many recipients.
Expert tips
If it's a critical document, consider sending it through a secure, dedicated system outside of standard email for maximum reliability.
For cold emails, avoid any attachments, as they can heavily trigger spam filters.
Think about the recipient's perspective: Is an attachment truly the most convenient way for them to access this information?
Remember that attachment policies can vary greatly between different email providers.
Expert view
Expert from Email Geeks says: PDF attachments definitely affect email delivery rates negatively.
2020-03-24 - Email Geeks
Expert view
Expert from Email Geeks says: Email clients see attachments as a potential red flag for spam due to the risk of malware.
2020-03-24 - Email Geeks
Key takeaways
While direct PDF attachments might seem convenient, their potential to negatively impact email deliverability, strain infrastructure, and create a suboptimal user experience is a significant concern for email senders.
The safest and most effective strategy for sharing documents via email is to host them on a secure server and provide a clear, trackable link within your message. This approach not only ensures better inbox placement but also allows you to gather valuable engagement data and maintain greater control over your content.
By moving away from direct attachments, you protect your sender reputation, enhance email performance, and provide a more streamlined experience for your recipients.
