How to find out what makes a PDF so big
See exactly what is taking up space in a PDF with PDF by Zaweba's Audit space usage, which shows bytes by kind, then shrink the file sensibly.
The quickest way to see what makes a PDF big is Audit space usage in PDF by Zaweba, found under File, Reduce file size. It breaks the file down into bytes by kind, so you can see how much is pictures, how much is fonts and so on. With that in front of you, you know which setting to change instead of guessing. The app is free and works offline on Windows.
It's a small tool that saves a lot of trial and error.
Running the audit
Open the PDF and choose File, Reduce file size, then Audit space usage. The app lists the kinds of content in the file with the bytes each takes. Look for the biggest share. In most large PDFs, one kind stands out.
Reading the results
If pictures dominate, the file was probably made with full-resolution photos or scans. That's the commonest cause by far. Reduce file size with a lower picture resolution and JPEG quality will help most.
If fonts take a large share, the document may embed many complete fonts. Document properties (Ctrl+D) has List the fonts, which shows each font's kind, whether it's embedded or a subset, and where it's used. That helps you see which font is responsible.
If the total seems far larger than the visible content explains, the file may be carrying things you can't see. Protect, Remove hidden information lists document details, XMP, attached files, comments, form fields, invisible text, content off the page, bookmarks, scripts and earlier versions, with counts. Attached files and earlier versions in particular can add a lot.
Earlier versions and incremental saves
Ordinary saves in a PDF are incremental. Each save adds the changed objects to the end of the file and keeps the old ones. A document saved many times during editing can carry several copies of its pictures. File, Earlier versions opens each incremental save in a tab of its own, which shows how many there are.
Save as writes a fresh file, leaving out unused objects, and can pack small objects into compressed streams. On a heavily edited document, that alone can make a big difference.
Acting on what you find
Once you know the cause, choose the fix. For pictures, Reduce file size with settings: pick a preset such as Balanced, Smallest file or High quality, and adjust resolution and JPEG quality. For hidden extras, remove what you don't need. For edit history, Save as. Then run the audit again on the new file to see the difference.
Work on a copy. Picture detail thrown away can't be put back, and you may want the full original for printing later.
When a big file is fine
Not every large PDF needs shrinking. A brochure going to a professional printer, or an archive of high quality scans, is meant to be large. The audit is still useful there: it confirms the size is going on the pictures you want, not on leftovers.
PDF by Zaweba does the audit and the shrinking on your own computer, without uploading the document.
