A text document of a few pages should be a few hundred kilobytes at most. When a PDF weighs 30 MB and nobody can tell why, something inside it is heavy. The good news is that a handful of causes explain nearly every oversized PDF, and each has a different cure. Work out which one you are dealing with first, and you can fix the size without wrecking the quality.
First, find out what is inside
You can guess a lot from how the file was made. Ask yourself:
- Was it scanned or photographed? Then it is mostly images.
- Was it exported from a presentation or design tool? Then it probably holds large pictures and many fonts.
- Can you select and copy the text? If yes, text is not the problem. If no, each page is a picture.
- How many pages are there, and how big is each page on average? Divide the file size by the page count. Under 100 KB per text page is lean. Several megabytes per page means heavy images.
Cause 1: High-resolution images
A photo taken on a modern phone has 12 million pixels or more. Dropped into a document and printed at a few inches wide, most of those pixels are wasted, yet the PDF keeps them. Ten such photos can add 30 MB.
Fix: recompress the images. Compress PDF re-encodes the pictures inside the file at a lower size and quality while leaving text sharp. Balanced is a good default. If you are building the document yourself, resize the photos before inserting them.
Cause 2: Scanned pages stored as full-size images
A scanned page is a picture of the whole sheet. At 300 DPI in color that is millions of pixels per page, and a 50-page scan can easily pass 100 MB.
Fix: compress the file, and next time scan in black and white or grayscale at 300 DPI. Our guide to scan settings explains which choices make the biggest difference. If a file is already scanned, the page-to-image option in Compress PDF gives the smallest result, at the cost of selectable text.
Cause 3: Fonts embedded in full
PDFs usually embed the fonts they use. Often only the characters needed are stored (a subset), but some exports embed complete font files, which can add a megabyte or more per font. Documents with many fonts and weights grow quickly.
Fix: use fewer fonts, and in your export options choose to embed subsets. Re-exporting from the source document is the cleanest solution. A compression tool will not usually shrink fonts.
Cause 4: Duplicate or leftover objects
Some programs save changes by appending them to the end of the file instead of rewriting it. Do this many times and the PDF carries old versions of pages and images that are no longer visible. Copying a logo onto each page can also store the same picture many times if the program does not reuse it.
Fix: save the document as a new file or use a tool that rebuilds it. Our tools write a fresh file from the pages, which tends to drop leftovers. Compress PDF also restructures the file for a small additional saving.
Cause 5: Transparency, layers and vector complexity
Maps, architectural drawings and charts exported with thousands of tiny vector shapes, shadows and transparencies can be very heavy, because each shape is described in detail. A page that looks simple may contain an enormous number of objects.
Fix: simplify before exporting, or export complex pages as a single flattened image if you can accept losing sharp zoom. For documents that must remain vector, accept the size and split the file for delivery.
Cause 6: Attachments and embedded files
PDFs can carry other files inside them: spreadsheets, original documents, even videos. They add size without being visible on the page.
Fix: check the attachments panel in your viewer. If you do not need them, export again without attachments. If you do, send them separately.
Cause 7: Too many pages in one file
Sometimes the file is simply long. A 600-page report is going to be large even when each page is lean.
Fix: split it into logical parts with Split PDF, for example one file per chapter, and send what the recipient actually needs. If a long scan is merely clumsy, remove blank pages with Remove Pages.
A quick decision guide
- Mostly images or scans: compress, then consider re-scanning in black and white.
- Photos you inserted yourself: shrink them before inserting next time, or compress the PDF.
- Many fonts or old edits: re-export from the source document.
- Too long: split into parts.
- Still too large: use the stronger compression levels, and accept a small quality loss.
When you build a PDF from pictures with JPG to PDF, remember that the images are embedded as they are, so reduce their size first if the target is small.
