Why Your PDF Is So Large, and How to Make It Smaller

Updated 2026-08-03 · 5 min read

A ten-page document has no business being 40 MB, yet it happens constantly. Almost always one of a few specific things is responsible, and identifying which one tells you the fix.

Images are usually the culprit

Text is remarkably compact. A page of text might be a few kilobytes. A single photograph straight from a phone can be several megabytes.

The common pattern: someone inserts full-resolution photos into a document. Those images display at perhaps 10 cm wide, but the file still stores every original pixel. A report with twenty photos becomes enormous while looking like a modest document.

The fix is to resize images before inserting them. An image displayed at 10 cm needs roughly 1200 pixels across for good print quality, not 4000.

Scans are images, and behave accordingly

A scanned document is not text at all; it is a picture of each page. Size is driven almost entirely by scan resolution.

Many scanners default to 600 DPI, which is far more than most documents need. For ordinary text, 300 DPI is the standard for print quality and 150 DPI is fine for on-screen reading. Dropping from 600 to 300 typically cuts the file to roughly a quarter.

Scanning in colour when the original is black and white also multiplies size for no benefit.

Embedded fonts

PDFs embed fonts so the document looks identical everywhere. That is the format working as intended, but a full font family can add hundreds of kilobytes.

Most tools embed only the characters actually used, called subsetting. Files created by older or unusual software sometimes embed entire fonts, and using many different fonts multiplies the cost.

Invisible leftovers

PDFs accumulate things you cannot see: previous revisions retained by incremental saves, cropped-away portions of images that are hidden but still stored, unused form fields, annotations, and metadata.

This is why a PDF sometimes grows after light editing. It is also a quiet privacy issue — cropping an image in some tools hides the cropped area rather than deleting it.

What to try, in order

1. Check what the size actually is. A text-only document that is 30 MB has a specific problem, not a general one.
2. Resize images before inserting and regenerate the PDF. This is by far the most effective fix.
3. Rescan at a lower resolution if the document is a scan.
4. Extract only the pages you need. Sending 5 pages instead of 80 is the simplest reduction available and often overlooked.
5. Re-save through a different tool, which frequently discards accumulated leftovers.

What to be careful with

Aggressive compression degrades scanned text badly. Small characters develop artefacts and become hard to read, and worse, unreadable to OCR later. For anything that must remain legible or searchable, prioritise resolution over file size.

Also keep the original. Compression is one-way: once detail is discarded you cannot restore it, and a document you may need to print or archive deserves an untouched copy.

If it is for email

Most mail systems cap attachments around 20–25 MB. Rather than compressing a document until it looks poor, consider sending only the relevant pages, or sharing a link instead. Recipients generally prefer a clean, readable document over a small, blurry one.