Summary
Attaching PDFs to emails presents a complex relationship with deliverability. PDFs are not inherently harmful, especially in transactional contexts, but many factors influence their impact. Sender reputation, PDF structure (including version and embedded links), file size, recipient expectation, and the type of email (promotional vs. transactional) all play critical roles. Exceeding size limits, sending unsolicited attachments, and having a poor sender reputation can trigger spam filters. Best practices recommend using links to hosted PDFs, scanning for malware, minimizing file sizes, and ensuring PDF structure adheres to older, simpler standards to avoid triggering security measures. Major platforms like Google Workspace and Microsoft Exchange also enforce attachment size limits.
Key findings
- Conditional Impact: PDF impact on deliverability is conditional, depending on various factors rather than being universally negative.
- Sender Reputation: Poor sender reputation increases the likelihood of PDF attachments being flagged as spam.
- PDF Structure Matters: Older PDF versions (1.4 or 1.6) without external links or embedded media are less likely to trigger spam filters.
- File Size Limits: Exceeding size limits set by email platforms (Gmail: 25MB, Exchange: 10MB default) can cause delivery issues.
- Attachment Suspicion: Email filters often treat attachments with suspicion, particularly in unsolicited emails.
- Promotional Emails: Attachments are especially discouraged in promotional emails, often negatively impacting deliverability.
- Scanning Limitations: Email security appliances (e.g., Cisco ESA) may limit the size of attachments they can scan, potentially causing delivery delays or failures for larger files.
Key considerations
- Use Links: Prioritize linking to hosted PDFs over direct attachments to reduce file size and avoid spam flags.
- Scan for Malware: Always scan PDFs for malware before sending to protect recipients and maintain sender reputation.
- Optimize PDF Structure: Use older PDF versions, avoid external links, and compress PDFs to minimize file size and complexity.
- Manage Expectations: Inform recipients about expected attachments to prevent them from marking the email as spam.
- Transactional Exceptions: PDFs may be more acceptable in transactional or CRM emails, but best practices should still be followed.
- Adhere to Size Limits: Stay within attachment size limits specified by major email platforms and consider alternative delivery methods (e.g., cloud storage links) for larger files.
- Evaluate Need: Assess if an attachment is truly necessary or if the information can be included directly in the email body to avoid potential deliverability issues.
What email marketers say
12 marketer opinions
Attaching PDFs to emails has a nuanced impact on deliverability. While not universally detrimental, many factors influence whether a PDF attachment negatively affects your email's success. Key considerations include sender reputation, the PDF's structure and content, file size, and recipient expectations. Best practices suggest using links to PDFs hosted online whenever possible, especially for marketing or unsolicited emails, to minimize potential deliverability issues and improve tracking.
Key opinions
- Sender Reputation: A sender with a poor reputation is more likely to have emails with PDF attachments flagged as spam.
- PDF Structure: The way a PDF is structured (e.g., version, embedded links, media) can impact deliverability. Older versions without external links are preferred.
- File Size: Large PDF file sizes can negatively impact deliverability, potentially causing delays or failed delivery.
- Recipient Expectation: Sending unsolicited attachments, especially PDFs, is risky and can damage sender reputation.
- Type of Email: Attachments, especially in promotional emails, are generally frowned upon and often decrease deliverability.
Key considerations
- Use Links Instead: Whenever possible, use links to PDFs hosted online instead of attaching the file directly to the email.
- Scan for Malware: Scan PDFs for malware before sending to avoid triggering spam filters.
- PDF Structure: Ensure the PDF is structured in a way that minimizes the risk of being flagged as spam (e.g., use older PDF versions, avoid external links).
- File Size: Keep PDF file sizes as small as possible.
- Recipient Awareness: Ensure recipients are expecting the attachment, especially when sending sensitive or important documents.
Marketer view
Email marketer from EmailToolTester advises that it's generally better to include a link rather than an attachment, as it avoids large file sizes and potential spam flags. Plus, you can track link clicks.
16 Dec 2021 - EmailToolTester
Marketer view
Email marketer from Web Hosting Talk Forum comments that in the context of promotional emails, attachments are generally frowned upon and will almost always hurt your deliverability rate.
13 Aug 2023 - Web Hosting Talk Forum
What the experts say
4 expert opinions
The impact of PDF attachments on email deliverability is varied. While PDFs aren't inherently problematic, filters often view them with suspicion, potentially flagging them as spam. Sending attachments, especially to large lists, can negatively affect sender reputation. If the PDF content is the same for everyone, it is often preferable to host the PDF online or include the content directly in the email body.
Key opinions
- Not Inherently Problematic: PDFs are not inherently problematic for email deliverability, particularly in transactional or CRM emails.
- Filter Suspicion: Email filters often treat attachments, including PDFs, with suspicion, which can lead to emails being flagged as spam.
- Sender Reputation Impact: Sending attachments, especially to large lists, can negatively affect sender reputation, thus harming deliverability.
- Content Alternatives: If the PDF content is the same for all recipients, consider hosting the PDF online and linking to it, or including the content directly in the email body.
Key considerations
- Attachment Necessity: Evaluate whether an attachment is truly necessary. Could the information be presented in the email body or via a link to a hosted document?
- List Size: Avoid sending attachments to large mailing lists unless absolutely necessary.
- Sender Reputation: Maintain a good sender reputation to reduce the likelihood of attachments being flagged as spam.
- Content Duplication: For common content, host the PDF online or include the information directly in the email body.
Expert view
Expert from Email Geeks shares that if the PDF content is the same for everyone, host the PDF so it renders, or better yet, put the content directly on the web and in the email.
26 Jul 2021 - Email Geeks
Expert view
Expert from Word to the Wise, Laura Atkins, responds that many filters treat attachments with suspicion and can flag them as spam. Senders should be aware of the potential impact on deliverability.
26 Apr 2023 - Word to the Wise
What the documentation says
4 technical articles
Email attachments, particularly PDFs, are subject to size limitations that can affect deliverability. While RFC 5322 doesn't specify a hard limit, exceeding reasonable sizes can cause delivery issues. Platforms like Google Workspace and Microsoft Exchange enforce attachment size limits (25MB and 10MB by default, respectively), requiring alternative methods like cloud storage for larger files. Cisco's Email Security Appliance also imposes size limits for scanning attachments, where exceeding them can increase spam scores and lead to delivery problems.
Key findings
- RFC 5322: RFC 5322 doesn't specify a maximum message size, but exceeding reasonable limits can cause issues.
- Google Workspace: Google Workspace limits attachments to 25 MB, recommending Google Drive for larger files.
- Microsoft Exchange: Microsoft Exchange defaults to a 10MB attachment limit, configurable by the administrator.
- Cisco ESA: Cisco Email Security Appliance imposes size limits for attachment scanning; exceeding these limits can increase spam scores and cause delivery issues.
Key considerations
- Attachment Size: Be mindful of attachment size limits imposed by email platforms and security appliances.
- Alternative Delivery: For files exceeding size limits, use cloud storage services (e.g., Google Drive) and share links instead of attaching the files directly.
- Admin Configuration: Administrators can configure attachment size limits on platforms like Microsoft Exchange to suit organizational needs, balancing security and usability.
- Scanning Limits: Be aware that email security appliances may have limitations on the size of attachments they can effectively scan, potentially affecting delivery.
Technical article
Documentation from RFC Editor explains that while RFC 5322 does not set a specific maximum message size, exceeding reasonable limits may cause issues with relaying and delivery.
18 Nov 2024 - RFC Editor
Technical article
Documentation from Cisco states that their Email Security Appliance has configurable limits on file sizes that can be scanned. Exceeding these limits leads to a higher spam score and potential delivery delays or failures.
30 Oct 2024 - Cisco
Are HTTP links penalized by spam filters in email marketing?
Are spam trigger words, PDF attachments, and links bad for email warm-up and deliverability?
Can linking to PDF files in email cause bounces due to Mimecast or other security filters?
Do images in email and PDF attachments affect email deliverability?
Do PDF attachments negatively impact email deliverability and what are the best practices?
Does linking directly to a PDF download cause email deliverability issues?
Does linking directly to PDFs from emails negatively affect deliverability?
