Does attaching a PDF to an email hurt deliverability?

Key findings

• Security risks: PDFs can be exploited for malware or phishing, leading to heightened scrutiny from email service providers (ESPs) and security software, which may impact your email deliverability.
• Recipient perception: Unexpected attachments, regardless of file type, often raise suspicion among recipients, increasing the likelihood of spam complaints.
• File size: Large PDF attachments can slow down email processing and delivery, potentially leading to timeouts or bounces, especially for marketing campaigns.
• Filter triggers: Some email marketing tools prevent attachments precisely because they can trigger spam traps and damage sender reputation.

Key considerations

• Context is key: Transactional emails, like invoices or tickets, where recipients expect an attachment, typically face fewer deliverability issues compared to marketing or cold emails.
• Sender reputation: Senders with a strong, established reputation might have more leeway, but even they should exercise caution. Regularly monitoring your domain reputation is a good practice.
• Technical structure: The internal structure and version of the PDF can influence how readily it is scanned and accepted by various inspection engines.
• Alternative methods: Linking to a hosted PDF or embedding relevant content directly in the email are generally safer and more effective alternatives, as highlighted by Mailjet.

Key opinions

• Spam folder risk: Many marketers believe that sending emails with PDF attachments significantly increases the chance of messages landing in spam folders, impacting overall email deliverability.
• Recipient suspicion: There's a common concern that unexpected attachments make recipients wary, potentially leading to lower engagement or higher complaint rates.
• Marketing platform limitations: Marketers frequently note that email marketing platforms actively discourage or prohibit direct attachments because of deliverability risks.
• Campaign performance impact: Large PDF attachments are seen as a factor that can slow down the sending of bulk email campaigns and increase loading times for recipients.

Key considerations

• Prior communication: If an attachment is necessary, it's advised to inform recipients beforehand to manage expectations and reduce suspicion.
• Content alternatives: Consider whether the PDF content can be reformatted directly into the email body or hosted on a webpage for better accessibility and deliverability. This is similar to considerations for image-only emails.
• User experience: Marketers prioritize a seamless user experience, and attachments can introduce friction, especially for users on mobile devices or with limited technical knowledge.
• Transactional vs. marketing: There's a distinction in practice, with many marketers accepting PDFs for transactional purposes (invoices, tickets) but largely avoiding them for promotional content, as advised by Campaign Refinery.

Key opinions

• Conditional impact: Experts agree that PDFs are not universally bad for deliverability. Their effect is contingent on multiple variables, including sender reputation and recipient behavior.
• Technical scrutiny: PDFs, due to their potential for embedded malicious content, undergo intensive scanning by security systems, which can sometimes lead to delivery delays or blockages, affecting overall email deliverability issues.
• PDF versioning: Older, simpler PDF specifications (like v1.4 or v1.6) are often preferred as they present a smaller attack surface and are more easily processed by a wide range of inspection engines.
• User expectation: The social aspect of email, specifically whether the recipient expects an attachment, significantly influences how mailbox providers and recipients react to the email.

Key considerations

• Content of the PDF: Even when linking to a PDF, the content within it (e.g., presence of suspicious links or media) can still be scrutinized by security scanners. This is a crucial aspect of understanding your email domain reputation.
• File integrity: Ensuring the PDF is clean, free of malware, and technically well-formed is critical, whether attached or linked.
• Domain reputation: A strong, consistent sender reputation can provide some mitigation against attachment-related filtering, though it's not a complete safeguard, as noted by EmailKarma.net. Maintaining a positive reputation is key to improving domain reputation.
• Alternative content formats: For complex instructions or general information, transforming the content into HTML within the email or on a landing page is usually the most deliverable and user-friendly approach.

Key findings

• Spam trap triggers: Documentation frequently identifies attachments as elements that can activate spam traps, which are designed to catch malicious or unsolicited email, impacting your placement on a blocklist.
• Increased filtering: Many systems are configured to apply higher scrutiny and more aggressive filtering rules to emails containing attachments due to security concerns.
• Performance overhead: Documentation notes that attachments, especially large ones, increase the data load and processing time for email infrastructure, potentially impacting delivery speed and success rates.
• Malware vector: PDFs and other common attachment types are explicitly cited as common vectors for malware and phishing attacks, leading to their cautious treatment by security systems.

Key considerations

• Transactional vs. promotional: There's a clear distinction in recommended practices, with transactional emails sometimes permitting attachments due to explicit user expectation.
• Plain text preference: Where attachments are unavoidable, documentation often implies or explicitly states that plain text files are the least risky, as mentioned by Higher Logic.
• Hosting and linking: The recommended alternative is almost universally to host files externally on secure servers and provide a link within the email, bypassing direct attachment risks.
• Content within files: Even if linked, the content of the document itself (e.g., embedded links to suspicious domains) can trigger warnings from security scanners that follow the link. This relates to how email deliverability issues are assessed.