How to Reduce PDF File Size for Email Attachments

Attachment limits are smaller than they look

Every email provider and many company mail servers limit the total size of a message. Public webmail services commonly allow somewhere around 20–25 MB (Gmail documents a 25 MB limit, for example), while business mail systems are frequently configured lower. The limit applies to the whole message, including all attachments and the message text.

There is a second, less obvious factor. Email was designed for text, so attachments are converted into a text-safe form called Base64 before they are sent. Base64 represents every 3 bytes of data with 4 characters, which makes an attachment roughly a third larger in transit. A 20 MB PDF therefore needs about 27 MB of message space.

Find out why your PDF is large

Before compressing, it helps to guess where the size comes from. The cause decides which approach works:

• Photos or screenshots in a report or brochure: usually the biggest and easiest win. Re-encoding images can reduce these files dramatically.
• A scanned document: every page is a large image. See the dedicated guide on compressing scanned PDFs.
• Mostly text, exported from a word processor: often already small. If it is still large, embedded fonts or leftover editing data may be the reason.
• Technical drawings, maps or complex charts: vector artwork can be heavy and is not something image compression can reduce.

Step-by-step: shrinking a PDF to email it

1. Check the current size (right-click the file and choose Properties on Windows, or Get Info on a Mac).
2. Open the SlimMyFile PDF compressor and select the PDF. The file stays on your device.
3. Choose Balanced and compress. Note the new size shown in the result card.
4. If it is still above your target, run the original again with High compression.
5. For scans only, consider scanned document mode at 100–150 DPI, accepting that the text will no longer be searchable.
6. Download the compressed file, open it once to check it looks right, then attach it.

Always compress from the original file rather than from an already-compressed copy. Starting from the original gives better quality for the same size.

If compression is not enough

Some documents simply cannot get small enough without becoming unreadable. Practical alternatives:

• Split the document and send it in several messages, for example appendices separately.
• Remove pages the recipient does not need, such as cover pages or blank separators.
• Shrink the photos before building the document. If you are creating the PDF yourself, compress or resize the images first, then export the PDF.
• Share a link from a cloud storage service you already trust instead of attaching the file. Consider who can access the link.

Mistakes to avoid

• Zipping the PDF and expecting a big reduction. PDFs are already compressed internally, so a ZIP usually saves very little.
• Sending an aggressively compressed contract or form that the recipient must print or sign. Use Balanced or Low for important documents.
• Uploading confidential documents to an unknown website to compress them. With a browser-based tool, the file is processed on your own device; our privacy guide explains how to check.
• Forgetting that digital signatures break when a signed PDF is modified. If a signature matters, send the original or ask the sender for a smaller version.

Frequently asked questions