How to compress a PDF to 100 KB for an upload form

Read the upload rules first

A form that says “PDF, maximum 100 KB” is asking about the file's size. Check what else it wants before you compress: some portals also ask for a single file, a certain number of pages, or a scan in colour. Getting under the limit only helps if the file is otherwise accepted.

Open Compress PDF, choose Target size and type the limit. Common limits are 100 KB, 200 KB, 500 KB and 1 MB, and these links open it with the size filled in: 100 KB, 200 KB, 500 KB and 1 MB. It is free and needs no account.

Compress to a size, step by step

1. Open the page for your limit and choose or drop the PDF.
2. Check Maximum file size shows the number you need, in KB or MB.
3. Leave Save pages as images if needed on, unless the text in the file must stay selectable.
4. Select Compress to 100 KB (or your size).
5. Open the result and zoom in on names, numbers and signatures to make sure they're readable, then upload it.

If the PDF is already under the limit, the tool tells you and leaves it alone.

What the tool tries, in order

It starts with the best quality and works down only as far as it has to:

1. Shrink the images inside the PDF. It tries images at up to 200, 150, 125, 100, 85, 72 and 60 DPI, with the JPEG quality lowered at each step, and keeps the best one that fits. Text and drawings aren't touched.
2. Save pages as images. If even 60 DPI images leave the file too big, the fonts and drawings are what's taking the space. Each page is then drawn as a single picture, at 150 DPI down to 40 DPI, until the file fits. The result says when this happened, because the text can no longer be selected or searched.

It aims under 100,000 bytes for 100 KB, and under 1,000,000 bytes for 1 MB. Sites disagree on whether a kilobyte is 1,000 or 1,024 bytes, and the smaller number passes both.

Why a scan fits more easily than a text PDF

A scanned page is already a picture, so shrinking it costs nothing but sharpness. In our tests, a PDF of three full-page colour images at 300 DPI, the size of A4 scans, went from 10.6 MB to 93 KB, with its images at up to 85 DPI.

A PDF made from Word can be the opposite: tiny images, but fonts embedded in full. When those fonts alone are larger than the limit, saving the pages as images is the only way to get under it.

If it won't fit

Every page needs a few kilobytes even at the lowest resolution, so long documents can't always reach 100 KB. If the tool says it couldn't get under the limit, it also says how small the file got. Then:

• Send only the pages the form asks for. Extract PDF Pages saves just those.
• Rescan in greyscale at 150 to 200 DPI instead of colour at 600 DPI, and crop off the scanner bed around the page.
• Split a long document with Split PDF if the form accepts several files.

Where your file goes

Everything happens in your browser: the PDF isn't uploaded to LabAnvil or anyone else. Keep your original. The compressed copy is right for the form, but not for printing or archiving.