How to reduce PDF file size for email and upload limits

Most email services cap attachments at around 20–25 MB, and many upload forms allow far less. When a PDF is too big, the fix depends on what is making it big — and for most large PDFs, that is pictures.

How small does it need to be?

Start with the target, because it decides how hard you have to squeeze. Job portals, government forms and university systems often set limits of 2, 5 or 10 MB and state them next to the upload button. Email is less obvious. Attachments are re-encoded for sending in a way that adds about a third to their size, so a 19 MB file can be refused by a 25 MB limit. For email, aim to stay under about 18 MB, and well under that if you do not know the limit on the receiving side.

What makes a PDF large

Text is tiny: a hundred pages of plain text is usually well under a megabyte. The weight comes from images — scanned pages, photographs, high-resolution graphics. A document scanned at high quality stores every page as a detailed picture, which is why a twenty-page scan can be larger than a thousand-page novel.

You can tell which kind you have in a few seconds. Open the PDF and try to select a sentence with the mouse. If individual words highlight, the file contains real text and is probably small per page already. If nothing highlights, or the whole page selects as one block, the pages are pictures — and pictures are what compression is good at.

Compress the PDF

1. Open Compress PDF and add your file.
2. Pick a level. Balanced suits most documents; Strong gives the smallest file for email; Light keeps the most detail for printing.
3. Press Compress and compare the before and after sizes before you download.

Compression works by re-drawing each page as an image at a lower resolution and quality. On scans and image-heavy brochures that typically removes 60–90% of the size. It has one trade-off to know about: the compressed pages are pictures, so their text can no longer be selected or searched.

A worked example

A 24-page contract scanned in colour comes out at 38 MB — too big for almost any inbox. Because it is a scan, it is exactly the kind of file that compresses well: a 60–90% reduction would bring it to somewhere between about 4 and 15 MB. Start with Balanced and look at the size it reports. If that is under your limit, open the result and read the smallest print on one page. If it is still too large, try Strong and check the small print again. The right level is the strongest one at which you can still read everything comfortably.

When compression does not help

A PDF that is mostly text is already stored efficiently. Turning its crisp text into page images can make it bigger, not smaller — the tool tells you when that happens and lets you keep the original. For those files, reduce what is in them instead:

• Remove pages nobody needs with Delete PDF Pages.
• Send it in parts: Split PDF can cut it every few pages or into ranges you choose.
• If you are building the PDF from photos, shrink the photos first with Compress Image — a phone photo rarely needs to be 4,000 pixels wide to be read on a screen.

Choosing a level

Check the result at the size it will actually be read. A document someone will skim on a laptop can take Strong compression; one that will be printed, or that contains small print or fine drawings, should use Light. Always open the compressed copy and read a page of the smallest text before you send it.

Keep the original file. Compression cannot be reversed — the detail that was removed is gone from the smaller copy — so treat the compressed PDF as the version you send and the original as the version you keep.

Make smaller PDFs in the first place

If you scan documents regularly, the scanner settings matter more than anything you do afterwards:

• Resolution. 150–200 dots per inch is enough for a text document that will be read on screen. 300 is the usual choice for printing or for archiving. Anything higher mostly adds megabytes.
• Colour mode. A page of black text does not need a colour scan. Greyscale is much smaller than colour, and black-and-white smaller again.
• Phone scans. Use the scanning mode in your notes or files app rather than the camera. It crops to the page and saves a far smaller image than a full photograph.

When you create a PDF from a word processor, look in the export dialog for a minimum-size or reduced-size option. It shrinks embedded pictures while leaving the text as real, searchable text — the best of both.

If it still will not fit

Some documents are simply large: a hundred-page scanned report will not go under 5 MB and stay readable. In that case, stop fighting the limit. Upload the file to a cloud storage service you already use and send a share link instead of an attachment, or split the document and send it as clearly numbered parts. If it is going to an upload form with a strict limit, ask the organisation what they would prefer — they meet this problem every day.