QiLing ToolsPDF Too Large? Identify the File Type, Then Pick a Method
Large PDFs get large for different reasons. Scans shrink best as compressed images, text PDFs need lossless optimization to keep selectable text, and the rest can be split by size.
Email attachments capped at 20 MB, upload forms that only take PDFs under 5 MB… the first instinct is to "just compress it". But there is no one-size-fits-all way to shrink a PDF. Pick the wrong method and the file barely changes, or the text turns into an image you can no longer copy or search.
Step 1: find out where the space goes
Open the file in PDF Info & Inspector to see what's inside: page count, embedded fonts, and how many images there are at what resolution. Or use a quicker test: can you select the text in a PDF reader?
- No, each page is one picture → it's most likely a scanned or image-based PDF.
- Yes → it's a text-based PDF, usually bloated by images, duplicated fonts or uncompressed data.
Scanned PDFs: recompress as images
Every page of a scan is already an image. Compress PDF re-renders each page at the resolution (DPI) and quality you choose, which typically shrinks the file by 50–90%.
Start at 150 DPI, which is plenty for on-screen reading, and go up to 200–300 DPI if you need to print.
Note that this turns pages into images, so text can no longer be selected or searched. A text-only PDF may even get bigger this way.
Text-based PDFs: optimize losslessly
For contracts, papers and ebooks where the text must stay intact, use Optimize PDF. It leaves every page looking exactly the same and slims down the file's structure instead:
- merges fonts and images that were embedded more than once (common after merging files or exporting page by page);
- recompresses uncompressed data streams with Flate;
- removes thumbnails, metadata and old objects nothing refers to anymore.
If the file contains photos far larger than they are displayed, you can also turn on downsampling for oversized images only; every other image stays untouched. A before/after breakdown by category shows exactly where the space went.
Still too big? Split it by size
Some files simply contain a lot. Rather than crushing the quality until it's unreadable, use Advanced PDF Splitter to split it by size into several files you can send separately.
Summary
| File type | Recommended method | Text selectable afterwards |
|---|---|---|
| Scanned / image-based PDF | Compress PDF (as images) | No |
| Text-based PDF | Optimize PDF (lossless) | Yes |
| Simply a lot of content | Split by size | Yes |
All of these tools run locally in your browser and never upload your file, so they're safe for documents containing personal information.