Best formats for email attachments

PDF is the most reliable format for sending documents by email. It preserves layout and fonts across all devices and operating systems, does not require the recipient to have a specific application to open it, and is widely accepted by email providers. If you are sending a form, invoice, letter, or report, PDF is almost always the right choice.

The practical reason is that a PDF looks the same to the recipient as it does to you. A word processor document depends on the recipient having the same fonts and a compatible application. A PDF carries its own layout with it.

If the PDF is too large, the usual causes are embedded images at print resolution and, in scanned documents, high scan DPI. Compressing the PDF addresses the first. For the second, a scan at 600 DPI is often four times the size of the same page at 300 DPI, with no meaningful gain for on-screen reading.

For photos and images, JPEG is practical for most purposes. It keeps file sizes manageable and is universally supported.

If you need a transparent background (for a logo, for example), PNG is the better option, though PNG files are generally larger than JPEG at equivalent dimensions.

Two specific cases come up constantly:

iPhone photos. Recent iPhones save in HEIC, which many Windows machines and some email clients will not open. Converting HEIC to JPG before sending avoids a reply asking you to resend.

Screenshots. A screenshot is text and sharp edges, which JPEG handles poorly. PNG is usually both clearer and, for a typical screenshot, not much larger.

ZIP archives are useful when you need to send multiple files at once, or when individual files are close to the size limit. Compressing a batch of images or documents into a single ZIP can significantly reduce total size and simplifies the recipient's download.

FormatOS includes a ZIP-for-email workflow built for exactly this, alongside general ZIP creation.

One caveat worth repeating from the compression guide: zipping files that are already compressed (JPEGs, PNGs, PDFs, MP3s) saves very little. ZIP still helps there, but as a container rather than a compressor. It turns twelve attachments into one, which is often the real problem you were solving.

Executables (.exe, .bat), scripts, and certain archive types, including ZIP files containing executables, are sometimes blocked by mail servers or spam filters. This is a security measure and is generally applied without notifying the sender in any useful detail.

When in doubt, rename or re-archive to ensure delivery. Be aware that some filters inspect archive contents rather than only the file extension, so renaming a ZIP that contains a blocked file type may not help.

Total attachment limits vary by provider; most allow between 10 MB and 25 MB, but the recipient's mail server may impose tighter limits. That second point is the one people miss: your provider accepting a 25 MB attachment does not mean the recipient's will.

There is also encoding overhead. Attachments are encoded for transport, which inflates them by roughly a third in transit. A file comfortably under a stated limit on your disk can exceed it on the wire. Corporate mail servers in particular are often configured well below consumer limits.

If a file is genuinely large, email may be the wrong channel. A link to cloud storage avoids the problem entirely and lets the recipient download at their own pace.

1. Convert the document to PDF, or the image to JPG.
2. Resize images to the dimensions actually needed before compressing.
3. Compress or compress the PDF.
4. If there are several files, package them into a ZIP.
5. Check the final size against the tightest limit involved, not the most generous one.