Why Is My PDF File So Large?

Work out what is taking the space

Divide the file size by the page count. A text-only PDF should be well under 100 KB per page. A page with a few photographs might reach 500 KB. If you are seeing several megabytes per page, the document is image-heavy — almost certainly scanned.

A second clue: try selecting text. If you cannot, every page is an image, which settles it immediately.

Cause 1: scanned at too high a resolution

This is the biggest single cause of oversized PDFs. Scanners often default to 600 dpi, which is appropriate for archival reproduction of photographs and wildly excessive for a printed letter. Each doubling of dpi roughly quadruples the data.

For text documents that will be read on screen or reprinted normally, 200 to 300 dpi is plenty. Rescanning at a sensible resolution frequently cuts the file by 80 percent or more, and is far more effective than any compression applied afterwards.

Cause 2: images embedded without compression

Exporting from design tools, or printing to PDF from an application, can embed images with little or no compression — sometimes storing a photograph as raw pixel data. A single such image can be tens of megabytes on its own.

Where the source document is available, resizing the images before export is the cleanest fix: a photo displayed at 10 cm wide does not need to be 6000 pixels across.

Cause 3: fully embedded fonts

PDFs embed their fonts so the document looks identical everywhere. Embedding a full font family rather than only the characters used can add a few megabytes. This matters mainly on short documents, where it can dominate a file that has no images at all.

Most modern PDF writers subset fonts automatically. If you are producing PDFs from a tool that does not, look for a font subsetting option in its export settings.

Cause 4: invisible leftovers

PDFs accumulate material you cannot see: previous revisions retained for incremental saving, cropped-away parts of images still stored in full, embedded file attachments, and unused form fields. A document edited many times can carry a great deal of this.

Exporting a clean copy — rather than saving over the original — usually discards it.

What actually reduces the size

One honest note: Filembly does not currently offer PDF compression. Meaningful compression requires re-encoding the images embedded inside the document, and we would rather not ship a tool that barely shrinks anything. Our split tool does help when the practical goal is simply getting under an email limit — removing the pages the recipient does not need is often the fastest route.

• Rescan at a lower dpi. Nothing else comes close for scanned documents.
• Downsample the images inside the PDF, which is what real compression tools do.
• Remove pages you do not need before sending.
• Export a fresh copy rather than saving over an edited original.

Getting a PDF under an email limit

Most mail providers cap attachments at around 25 MB, and many corporate systems are stricter at 10 MB. If you are close, splitting out the relevant pages is quicker and more reliable than compression, and the recipient usually prefers the shorter document anyway.

Well over the limit with a scanned document? Rescanning at 200 dpi will almost certainly do more than any other single action.