Why Is My PDF So Large? 8 Causes and How to Fix Each

Text is tiny. A plain-text novel of 300 pages fits comfortably in a PDF of 1 or 2 MB, while five pages scanned in colour can easily pass 20 MB. When a PDF is unexpectedly large, the cause is almost always images, and occasionally something the app that made the file tucked inside it. Finding the cause first saves you from compressing a file that didn't need it, or ruining one that did.

A 30-Second Diagnosis

1. Divide the file size by the number of pages. Under 100 KB per page is normal for text documents. Over 1 MB per page almost always means scanned pages or large photos.
2. Try to select a word. If you can't, the page is an image (a scan or a photo), and image compression will help a lot.
3. Think about where the file came from: a scanner, a phone camera, Word, a design app, or a PDF someone has edited many times. Each points to a different cause below.

1. Pages Scanned at a High Resolution

Scanners often default to 300 or 600 DPI in full colour. One A4 page at 300 DPI in colour is about 2,480 × 3,508 pixels, which is roughly 26 MB before any compression. Even after the scanner compresses it, colour scans commonly land between 0.5 and 3 MB per page.

Fix: if you can rescan, use 200 to 300 DPI and grayscale for text. Otherwise, compress the PDF: scans are the case compression handles best. Our guide to scanning documents to a small PDF covers the best scanner settings.

2. Full-Resolution Phone Photos

Modern phone cameras take photos of 12 megapixels or more, often 3 to 5 MB each. Drop a few into a Word document or a PDF maker and they are embedded at full size, even if they appear as small thumbnails on the page.

Fix: resize photos before inserting them, or use Word's Compress Pictures option before exporting. For an existing PDF, compression brings them down to a size that suits the page.

3. Fully Embedded Fonts

PDFs embed fonts so they look the same everywhere. Most apps embed only the characters you used (a subset), which is small. Some settings embed the whole font file, and large fonts, especially those covering Chinese, Japanese or Korean characters, can add several MB each.

Fix: in the exporting app, look for an option to subset fonts or to embed only the characters used. This fix belongs at the source; a compressor won't target fonts specifically.

4. Editing Data Saved Inside the PDF

Design apps can save a full editable copy of the original artwork inside the PDF so you can reopen it later. Adobe Illustrator's Preserve Illustrator Editing Capabilities option is the best-known example, and it can double or triple the file size.

Fix: turn that option off when you export a copy to share, and keep the original design file separately.

5. A Long History of Edits

Many PDF editors save changes by adding them to the end of the file rather than rewriting it. Every save adds another layer, and content you deleted can stay in the file. A form that has been filled in, signed and annotated by several people can grow well beyond its original size.

Fix: use Save As (not Save) to write a fresh copy, or run it through a tool that rebuilds the file. Merging, splitting or extracting pages in PDF Perfect all write a new, clean file. This also matters for privacy, as our PDF privacy checklist explains.

6. Detailed Vector Drawings

Maps, CAD drawings, engineering plans and charts with thousands of data points are stored as vectors: every line and point is described individually. They stay sharp at any zoom, but a complex drawing can be many megabytes.

Fix: if the drawing will only be viewed on screen, simplifying it at the source (fewer points, merged layers) is best. Compressing it turns the drawing into an image, which makes the file smaller but loses the sharp zoom.

7. Many Merged Files That Repeat the Same Resources

When you merge PDFs that came from different places, each one brings its own fonts and images. Ten documents using the same logo can carry ten copies of it.

Fix: this rarely adds more than a few MB. If the merged file is too big for an upload or email, compress it once after merging, or split it by size so each part fits.

8. Attachments and Embedded Files

PDFs can carry attached files, such as spreadsheets, other PDFs or videos. Your viewer may show them in a paperclip or attachments panel, and they count towards the file size even though they don't appear on any page.

Fix: open the attachments panel in your PDF viewer and remove anything the recipient doesn't need. Extracting the pages you want into a new PDF also leaves document-level attachments behind.

When Compression Is the Right Answer

PDF Perfect's compressor redraws every page as an optimised image. That's ideal for scans and photo-heavy PDFs, where it can cut the size by more than half. For text documents, it makes text unselectable and can make small print softer, so fixing the cause at the source (usually the images) is better.