Compress a PDF for email attachment limits

Attachment limits are hard limits. Gmail allows up to 25 MB of attachments per message. Outlook.com allows 20 MB. Company mail servers and upload portals often set their own, lower numbers, and you usually find out when the message bounces or the form rejects the file. If a portal states a limit, that is the number to plan for.

The real limit is smaller than it looks

Email was designed for text, so attachments are converted to base64 before they travel. Base64 turns every 3 bytes into 4 characters, which makes an attachment about 33% larger in transit. Many mail servers apply their limit to that encoded message, not to the file sizes you see on disk.

Take the test scan used below. On disk it is 18.9 MB (19,803,320 bytes). Encoded, it becomes about 26.4 million bytes, roughly 25.2 MB, which is over Gmail’s 25 MB even though the file itself is well under it. When you are close to a limit, assume the encoded size counts. A safe rule is to keep a single attachment under about three quarters of the stated limit: around 18 MB for Gmail and 15 MB for Outlook.com.

Upload portals are different: a web form usually checks the file size itself, with no base64 step, so a 10 MB portal will accept a 9.5 MB file.

How much each level saves: a measured example

The test file is a 10-page US Letter scan of a typed lease, stored as 300 ppi color JPEGs, the kind a flatbed scanner writes by default. Results from Compress PDF:

On this scan, Low already clears Gmail and Outlook.com with room to spare, and Recommended gets under a typical 5 MB portal limit. High was not needed for any of them. On a scan, High costs little because there was never selectable text; on an exported document it would remove the text layer, so treat it as a last resort.

Which level to try first:

• The file is only a little over the limit, and quality matters (printing, small text): Low.
• Most scans going by email: Recommended.
• A portal still refuses the Recommended result, and the document is a scan: High.

Text-only PDFs are already small

A 12-page report exported from a word processor was 35.3 KB in our test, and compressing it saved 4 KB. If a text document is over an email limit, something inside it is large, usually an embedded photo, a scanned signature page, or a high-resolution logo stored as separate copies. Compress will shrink those images. It will not do much for the text.

When to split instead

Sometimes the right answer is fewer pages, not smaller pages. If the recipient needs only the signature page and one schedule, Split PDF can extract them. Two things we measured are worth knowing first:

• A page that uses a shared image keeps that image. Our 40-page report, with the same 1.44 MB banner photo on every page, was 1.49 MB. The one-page extract was 1.46 MB. Splitting alone did almost nothing for size. Compressing that extract on Recommended brought it to 167 KB.
• Splitting does not reduce quality. It copies pages without resampling anything. So split first, then compress only what you are actually sending.

For a long scan with no shared images, sending it as two messages (pages 1 to 20, then 21 to 40) is a reasonable fallback when you need to keep full quality.

Several attachments in one message

The limit applies to the whole message, so three 8 MB attachments count as 24 MB before encoding. Compress each one, or send them separately. Combining them with Merge PDF makes the email tidier, but it does not make it smaller: a merge copies pages as they are.

When a link is the better answer

For very large scans, such as a 200-page records request or a set of drawings, squeezing the file under an email limit can cost more quality than you want to give up. A link to a file in storage you already use and control (your own Google Drive, OneDrive, or Dropbox, shared with the specific recipient) avoids the attachment limit entirely. That is an upload, but to an account you chose, with sharing you can revoke.

Checklist

1. Find the limit: Gmail 25 MB, Outlook.com 20 MB, portals as stated on the form.
2. For email, aim for about three quarters of the limit.
3. Remove pages the recipient does not need.
4. Compress on Recommended, then check the smallest text at 200%.
5. Use High only for scans, and only if Recommended is still too large.

How we measured

Sizes were measured on 27 September 2026 with this site’s Compress and Split tools in Chrome on a Linux desktop, using test files generated with ReportLab and Pillow. MB figures are as the tool displays them (1 MB = 1,048,576 bytes). The “size in an email” column is calculated at 4/3 of the file size and ignores the small extra overhead of message headers.