Upload forms give you a number, not a method. The right approach is different at 1 MB than at 100 KB, and below about 30 KB nothing works.
Starting points by target
| Target | PDF (scanned) | JPEG photo |
|---|---|---|
| Under 5 MB | 200 DPI, q0.8 | 2400px, q0.85 |
| Under 2 MB | 175 DPI, q0.78 | 2000px, q0.82 |
| Under 1 MB | 150 DPI, q0.75 | 1600px, q0.8 |
| Under 500 KB | 120 DPI, q0.72 | 1400px, q0.78 |
| Under 200 KB | 100 DPI, q0.7 | 1200px, q0.78 |
| Under 100 KB | 85 DPI, q0.65 | 900px, q0.75 |
| Under 50 KB | Split the document | 700px, q0.7 |
These are first attempts for a typical document or photograph, not guarantees. A page dense with photographs runs larger; a page of text on white runs much smaller. The point is to start close rather than binary-searching from 100%.
Dimensions before quality, always
Both levers shrink the file and they do different damage.
Reducing resolution removes pixels you were not going to display. At 150 DPI on a screen you cannot see what 300 DPI was holding, so the loss is invisible. Reducing quality keeps every pixel and describes each one more crudely — ringing at edges, blotching in flat areas, visible at any size.
So exhaust the resolution lever first. People who go straight to the quality slider end up at 0.35 with a smeared document, when 0.75 at half the resolution would have been both smaller and better.
When the limit is impossible
Some forms ask for what cannot be delivered, and recognising that early saves twenty minutes of sliding.
A 20 KB limit on a colour photograph is one. A multi-page scanned document under 100 KB is another — that is roughly 10 KB per page, below the point where text stays legible. And "300 DPI, under 100 KB" is self-contradictory for anything larger than a postage stamp.
- Try greyscale. If colour is not required, dropping the two chroma channels saves 20–25% for free.
- Send fewer pages. A limit that is impossible for 20 pages is comfortable for 4. Split and send what was asked for.
- Test PNG for documents. For a scan with a white background, PNG is sometimes smaller than JPEG and lossless.
- Re-read the requirement. Forms frequently state a limit for a different upload field, or state KB meaning something else.
If none of it works, the honest answer is that the form wants a smaller file than the content can be, and a tight crop at moderate quality beats a full page at quality 0.3.
Never compress an output
This is the mistake that costs the most quality and it is built into how people work.
Compress, check, still too big, compress the result again. Each lossy pass re-analyses artifacts from the previous one and treats them as detail worth preserving badly. Three passes at 0.8 look worse than one pass at 0.6 — and produce a larger file.
Keep the original untouched and go back to it for every attempt. It costs nothing extra and the result is measurably better. The same reasoning, with the DPI numbers, is in how to compress a PDF without losing quality.
Offline and built-in options
Several tools you already have will do this, with different trade-offs.
macOS Preview has a Quartz filter — Export, then Quartz Filter, Reduce File Size. It is aggressive and gives no control, so it often overshoots into visibly poor quality. Fine for a quick email attachment, wrong for anything that matters.
Microsoft Word can compress images before exporting: select an image, Picture Format, Compress Pictures, and choose a resolution. Doing it before Save As PDF is better than compressing the PDF afterwards.
Print to PDF as a crude reducer works, and discards hyperlinks, bookmarks and accessibility tags along the way.
A browser-based compressor has one advantage worth naming for this particular job: it shows the resulting size before you download, so you can adjust and re-check without a round trip. The documents people squeeze under form limits — ID scans, bank statements, medical letters — are also the ones worth not uploading.
What is actually large
Before compressing, it is worth thirty seconds to check the size is where you think.
Divide the file size by the page count. Under 100 KB per page is a normal text document and compression will barely help. Over 1 MB per page for something that looks like text means images, unsubsetted fonts or retained revision history — and only the first responds to compression.
A 12-page "text" document at 30 MB usually has a full-page background image behind every page. Compressing helps, but re-exporting from the source fixes it outright. The diagnosis is in why is my PDF so big.
Frequently asked questions
How do I compress a PDF to exactly 1 MB?
Start at 150 DPI and quality 0.75, which lands close for a typical scanned document, then adjust. Use a tool that shows the output size before you download so you are not guessing. Always work from the original rather than re-compressing a previous attempt.
Should I reduce resolution or quality?
Resolution first, always. Removing pixels you were never going to display is invisible; reducing quality damages the whole image at any size. Only use the quality slider to close the gap after resolution has done what it can.
Why does my file get worse each time I compress it?
Because lossy compression compounds. Each pass treats the previous pass's artifacts as real detail and damages them further. Three passes at 0.8 look worse than one at 0.6 and produce a bigger file. Go back to the original every time.
What if the size limit is impossible?
Try greyscale for a free 20-25%, send fewer pages, and test PNG for white-background scans. If none works, the form wants a smaller file than the content can be — a tight crop at moderate quality beats a full page at quality 0.3.
Is Preview's Reduce File Size any good?
It works and gives no control, so it frequently overshoots into visibly poor quality. Fine for a quick email attachment, wrong for anything that matters. A tool with an adjustable DPI and a live size readout gets you a much better result at the same size.
Should I compress in Word before exporting?
Yes — Picture Format, Compress Pictures, choose a resolution, then Save As PDF. Compressing the images at source is one lossy pass. Exporting at full resolution and then compressing the PDF is two.