A PDF that balloons to tens of megabytes is almost never a mystery once you know where to look. Bloat comes from a predictable short list of causes, and each one has a matching fix. Here are the seven usual suspects, roughly in order of how often they are to blame.
1. High-resolution scanned pages
A page scanned at 600 DPI holds four times the data of the same page at 300 DPI, and screen and print rarely need more than 150 to 200 DPI. Scans are the single most common cause of a heavy file.
2. Full-resolution photos
Dropping phone photos straight into a document embeds images far larger than the page can display. A 12-megapixel photo shown at postcard size is mostly wasted data.
3. Embedded fonts
PDFs embed fonts so they render everywhere. Usually fine, but a document that pulls in several full font families, especially large non-Latin ones, carries noticeable extra weight.
4. Repeated images and logos
A letterhead logo placed on every page can be stored dozens of times if the file was not built carefully, multiplying its size across the document.
5. Leftover editing data
Some editors keep hidden layers, revision history, or cropped-but-not-deleted image areas. You cannot see them, but they inflate the file.
6. Vector artwork with thousands of points
Complex charts and maps exported as vectors can contain enormous numbers of individual points, each one adding up.
7. Unoptimised structure
Files assembled by merging many sources sometimes carry duplicated resources and an inefficient internal layout that a good optimiser can tidy.
The takeaway
Before you accept a giant file, ask what is inside it. Nine times out of ten the answer is "images bigger than they need to be," and that is the easiest thing in the world to fix.
