How to Compress a PDF to 100 KB (or Any Size Limit)
By Tooldesk team · · 4 min read
To get a PDF under 100 KB, compress it with a strong setting that turns each page into an optimized image. Open our Compress PDF tool, add your file, choose Strong, and check the new size before you download. A one- or two-page scanned document usually drops below 100 KB this way. If it doesn’t, remove pages you don’t need, or lower the scan quality at the source.
The rest of this guide explains why some PDFs are hard to shrink, and what to try for each case.
Why upload forms ask for 100 KB
Job portals, university admissions and government sites often cap uploads at 100 KB, 200 KB or 500 KB. They set strict limits to keep storage small and uploads fast on slow connections. The limit applies to the whole file, so a three-page PDF has to fit the same budget as a one-page one.
Step 1: Check what is making the file big
Open the PDF and look at it:
| What you see | Likely cause | Best fix |
|---|---|---|
| Photos of pages from a phone or scanner | Each page is a large image | Compress with Strong |
| Mostly typed text, few images | Embedded fonts or extra pages | Remove pages; text PDFs are usually small already |
| Pages with colour photos or graphics | High-resolution images | Compress with Recommended, then Strong if needed |
| Many pages | Size adds up per page | Split out only the pages you must upload |
A phone photo of a page is often 1–3 MB. Saved into a PDF without changes, that is the size you’ll see.
Step 2: Compress the PDF
- Open Compress PDF.
- Add your file. It stays on your device and is not uploaded.
- Choose a level:
- Light keeps the most detail, which suits printing.
- Recommended suits most forms and email.
- Strong gives the smallest files, which suits 100 KB and 200 KB limits.
- Select Compress PDF and read the before-and-after sizes.
- Download the file if it fits. If it doesn’t, move on to Step 3.
The tool redraws each page as an optimized image. That is why it shrinks scans so much. The trade-off is that you can no longer select or copy the text in the result. For an upload form, that is almost always fine.
Step 3: If it is still too big
Upload fewer pages. If the form asks for an ID card, upload the page with the ID only. Use Split PDF to extract it, then compress that smaller file.
Rescan at a lower setting. Scanners often default to 300 or 600 DPI in colour. For a form, 150 DPI in greyscale is usually readable and several times smaller. On a phone scanner app, pick “black and white” or “document” mode.
Crop the margins. Large white borders still cost bytes in a photo. Crop to the page edges before you make the PDF.
Check whether an image is accepted. Some portals accept JPG as well as PDF. If yours does, convert the page to JPG, then compress the image to the limit.
How small can a page realistically get?
These are rough estimates for one A4 page at our Strong setting. Your results will vary with the scan quality, so always check the size the tool shows you:
| Page type | Rough size after Strong |
|---|---|
| Typed letter, scanned in black and white | 40–80 KB |
| Typed letter, scanned in colour | 60–120 KB |
| ID card or certificate with a photo | 80–150 KB |
| Full-page colour photo | 120–250 KB |
So a 100 KB limit is realistic for one or two text pages. For more pages, you’ll need fewer pages or a lower scan quality.
Other ways to compress a PDF
On a Mac: open the PDF in Preview, choose File → Export, and set Quartz Filter to Reduce File Size. It works well on image-heavy files but gives you no control over the level.
In Microsoft Word: if you made the document yourself, choose File → Save As → PDF and select Minimum size (publishing online). Starting from the original document beats compressing an existing PDF.
Adobe Acrobat: the paid version has a Reduce File Size option with detailed settings.
Questions people ask
Will compressing make my document unreadable?
Not at Recommended. At Strong, small print can get soft. Zoom in to check that names, numbers and signatures are clear before you upload.
Can I compress a password-protected PDF?
Not until the password is removed. Open it with the password, then save or print it as a new PDF without one.
Is it safe to compress private documents online?
It depends on the site. Many online tools upload your file to their servers. Ours runs in your browser, so your file is never sent anywhere.
Why did my file get bigger?
That happens with PDFs that are already mostly text, because turning text into images adds bytes. Keep the original, or remove pages instead.