How to Reduce a PDF's Size So It Fits an Email Attachment Limit

The limits you are actually working with

Every mail service sets a ceiling on how big a message can be, and the attachment has to fit inside it together with the text of the email. The figures below come from each provider's own help pages. They change from time to time, so the linked page is the one to trust if it disagrees with this table.

Service	Stated limit	What happens above it
Gmail (personal account)	25 MB for sending (Google)	Gmail swaps the attachment for a Google Drive link
Gmail (work or school)	Set by the Google Workspace administrator	Ask your IT team
Outlook.com	Microsoft quotes both 20 MB and 25 MB on different pages (20 MB, 25 MB)	Share from OneDrive instead; OneDrive attachments can be up to 2 GB
Classic Outlook desktop, internet account (POP/IMAP)	20 MB default in Outlook 2013 and later (Microsoft Learn)	Error: the file is bigger than the server allows
Outlook with a business Exchange mailbox	10 MB by default, but the administrator can change it	Same error; the number depends on your organisation
Yahoo Mail	25 MB for all attachments in one message, sent or received (Yahoo)	The message cannot be sent

Two practical rules follow from this. First, when Microsoft's own pages disagree, plan for the smaller number: a file that fits under 20 MB fits everywhere in the Outlook family that uses the default settings. Second, your limit is only half the story. The message also has to be accepted by the recipient's server, and if they work at a company running Exchange on default settings, that may be 10 MB. When you do not know where the email is going, a target under 10 MB is the safe one.

Why a 24 MB file can fail a 25 MB limit

Email was designed for text, so attachments are converted into text before they travel. The standard method, Base64, turns every 3 bytes of the file into 4 characters (RFC 2045). That makes the attachment about a third larger in transit than on your disk. Providers do not always say whether they measure the file before or after this conversion, but if a limit is applied to the encoded message, the arithmetic is unforgiving: a 25 MB cap holds about 18.75 MB of original file, a 20 MB cap about 15 MB and a 10 MB cap about 7.5 MB. Aim below those numbers and you never have to find out which way your provider counts.

What makes a PDF heavy in the first place

Before you shrink anything, it helps to know which kind of PDF you have, because the right fix depends on it.

• Scans and phone captures. Each page is a photograph of paper. A modern phone camera produces around 12 million pixels per shot (4000 × 3000), and an office scanner set to 300 dpi produces about 8.7 million for an A4 page (2480 × 3508). Multiply that by 30 pages and the file easily runs past 50 MB, even though the content is just typed text.
• Documents with large photos. A brochure or report exported from Word or PowerPoint often carries every picture at the resolution it was taken, even if it is shown at the size of a postage stamp on the page.
• Embedded fonts. A PDF that uses several unusual typefaces carries those fonts inside it. This adds hundreds of kilobytes rather than megabytes, so it is rarely the reason a file fails an email limit.
• Purely digital text. A 40-page contract exported straight from a word processor is usually well under 1 MB. If a text-only PDF is huge, the cause is almost always an embedded image, such as a scanned signature page or a high-resolution logo repeated on every page.

A quick test tells you which group you are in: open the file and try to select a sentence. If the cursor highlights words, the text is real text. If it draws a box over a picture, the page is an image, and image pages are where size reductions come from.

Choosing a Compress PDF level

Compress PDF rebuilds each page as a JPEG at a lower resolution. That is extremely effective on scans and photo-heavy files and counterproductive on text-only PDFs, which can come out larger than they went in. It also means the compressed file has no selectable text, so it is best used on scans, where there was no real text to lose. The three levels differ in two settings at once: the resolution each page is redrawn at and the JPEG quality.

Level	Resolution	JPEG quality	A4 page in pixels	Height of 8 pt text
Low	about 162 dpi	0.82	about 1340 × 1894	about 18 px
Recommended	about 124 dpi	0.65	about 1026 × 1450	about 14 px
Strong	about 97 dpi	0.5	about 802 × 1134	about 11 px

The last column is the useful one. An 8-point line, the size of typical footnotes and small print on forms, is 8/72 of an inch tall, so at 124 dpi it gets roughly 14 pixels and at 97 dpi roughly 11. Fourteen pixels is comfortable on screen; eleven is readable for normal letters but starts to blur figures like 3, 5 and 8 and thin strokes in stamps and signatures. So the sensible order is:

1. Start with Recommended. On a phone capture of an A4 page it cuts the pixel count from about 12 million to about 1.5 million, roughly an eighth, before the JPEG quality setting takes off more.
2. If the result is under your target and the small print is still sharp at 200% zoom, you are done.
3. If the result is still too big, try Strong, then check the smallest numbers on the page very carefully. Account numbers and dates matter more than how the page looks overall.
4. If Strong is not enough either, stop compressing and split the file or share a link. Going lower again would make the document hard to read for the person receiving it.

Two extra savings are worth knowing. A colour scan of a black-and-white document carries colour information that adds nothing; running Grayscale PDF before compressing removes it. And if only part of a file is scanned, for example a typed report with three signed pages at the end, compress only the scanned part and merge it back, so the typed pages keep their real text.

Splitting by size when one file will not do

Some documents cannot be squeezed under the limit without becoming unreadable: a 120-page scanned case file, an engineering drawing set, a photo-heavy inspection report. Splitting is then the honest answer, and it beats sending a blurry file.

Split by Size takes a size per file and fills each part with as many whole pages as fit under it, measuring the actual saved size after each page. A few points make it work smoothly for email:

• Set the number below your real limit. The tool counts 1 MB as 1,048,576 bytes. For a 25 MB service where you also want to allow for encoding, a value of 18 keeps you clear; for an unknown recipient, 7 is a reasonable choice.
• Send one part per email. Three attachments of 8 MB each in a single message add up to 24 MB, which fails exactly as the original would have. The limit is per message, not per file.
• Label the parts. Put "Part 1 of 3" in each subject line so the recipient knows when everything has arrived and in what order to read it.
• Watch for one oversized page. A page is never cut in two, so a single huge page, such as a full-colour poster scanned at high resolution, ends up alone in a part that is still too big. Compress that file first.

If the document has natural sections, such as chapters or one invoice per page, splitting on those boundaries with Split PDF gives parts that make sense on their own, which recipients usually prefer to arbitrary cuts.

Sharing a link instead of attaching

For files that are large because they need to be, a link is often better than any amount of compression. Both big providers build this in: Gmail turns an oversized attachment into a Google Drive link automatically, and Outlook.com can attach a file from OneDrive, which Microsoft says can be up to 2 GB.

The catch is permissions. In Google Drive, a file shared as Restricted can be opened only by people you have added by email address; Anyone with the link lets anybody who gets hold of the link view it without signing in (Google Drive help). For a scanned passport, bank statement or medical report, add the recipient by address rather than switching on public link access, and remove access once they have downloaded it. Also remember that links can be blocked: some company mail systems and some application portals will not accept a link where they expect an attachment, so check what the recipient needs before choosing this route.

Checking the result before you hit send

A file that fits but cannot be read wastes more time than one that bounces, because nobody tells you until days later. Spend two minutes on these checks:

1. Read the size from your file manager, not from memory. Compare it with the encoded limits worked out above, not only the headline figure.
2. Zoom to 200% on the worst page: the one with the smallest print, a faint stamp or a handwritten signature. If you squint, the recipient will too.
3. Check every page is there and upright. Count the pages against the original, especially after splitting.
4. Decide whether text needs to be searchable. A compressed file is a set of images. If the recipient will need to search or copy from it, run OCR PDF on the compressed copy to add an invisible text layer. Recognition is good on clean print and weak on handwriting, so do not rely on it for handwritten notes.
5. Keep the original. Compression throws detail away for good, so keep the full-quality file in case someone later asks for a sharper copy.

A short decision path

If the file is a scan or full of photos, compress it at Recommended and check the small print. If it is still too big, try Strong or convert to grayscale first. If that hurts legibility, split it by size and send the parts in separate emails, or share it through Drive or OneDrive with access limited to the recipient. If the file is digital text and still large, look for one heavy image page rather than compressing the whole thing. Throughout, size for the smallest limit on the route, which is often the recipient's server rather than yours.