Make a PDF smaller without wrecking the scan
Where the megabytes actually are, which lever to pull first, and when a file is already as small as it is going to get.
A PDF that will not send is nearly always a PDF full of pictures. Text is tiny. A hundred pages of prose is well under a megabyte. A single phone photo of one page can be four.
So before compressing anything, find out which kind of file you have.
Where the size is
| What the file is | Typical size | What to do |
|---|---|---|
| Typed document, text and vectors | Under 1 MB | It is already small. Compression will not help much. |
| Scan or photographed pages | 2 to 10 MB a page | Compress. This is the whole problem. |
| Export from design software | Varies | Often carries image data at print resolution. Compress. |
Pull the image lever first
Compress works on the images inside the file: it lowers their resolution to something sensible for the page size and re-encodes them. That is where the savings are, and it is also the setting that can go too far.
A rough guide:
- For reading on a screen, 150 dpi is plenty. A ten megabyte scan usually lands near one.
- For printing, stay at 300 dpi and accept a smaller saving.
- For a scan someone has to sign or a form with small print, check the result before you send it. Text that was already faint is the first thing to go.
Always keep the original. Compression discards detail, and there is no way back from the compressed copy.
When compression is the wrong tool
Sometimes the file is not too big, it is too long. If you only need pages 4 to 9 of a bank statement, extract those pages and the size problem disappears along with the pages nobody asked for.
Likewise, a file that is mostly one enormous embedded image of a signature page is better fixed by replacing that page than by squeezing the whole document.
A note on the limit
Everything happens in your device's memory, so a very large scan on a small phone can run out of room. The tool will say so rather than fail quietly. On a laptop the practical ceiling is much higher than anything email will accept anyway.
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