How to compress a PDF to 100 KB (and when it can't be done)

The size of the problem

A single colour page from a phone camera or flatbed scanner, saved inside a PDF, is normally 500 KB to 2 MB. Getting that down to 100 KB means removing 90 to 98 percent of the data — not trimming, discarding almost all of it. That is only realistic if most of what gets discarded is information the reader never needed: colour on a black-and-white document, resolution far beyond what text needs, pages nobody asked for.

Who actually asks for exactly 100 KB? Mostly older government portals, some exam-board application systems, and a handful of visa or licensing forms with a hard byte cap written into the upload field. If your form just says "keep it small" without a number, you likely have more room than 100 KB — check before you fight for the last kilobyte.

Step 1: The page count sets your real budget

The limit is shared across every page in the file:

Ten KB per page is roughly what a very small, low-resolution thumbnail needs — not a readable scan. In practice, a 100 KB limit only works comfortably for one or two pages. If your document is longer, your first and most effective move is cutting pages, not compressing harder.

Remove blank backs from double-sided scans, cover sheets, and any page the form doesn't actually require. Rotate & reorder lets you delete pages visually; Split extracts only the pages you name. Neither reduces the quality of the pages you keep.

Step 2: Strip colour if the content doesn't need it

A colour scan carries three channels of data per pixel; for black-and-white text, two of them are close to noise. A greyscale version of the same page is typically a third to half the size before any other compression is applied. If you still have access to the original paper document, rescan it in greyscale or black-and-white mode rather than converting a colour scan afterward — the quality difference is noticeable.

Step 3: Accept a lower resolution than you would for 300 KB

At 100 KB per page, legible text usually means somewhere between 60 and 90 dpi — lower than the 100–150 dpi range that works for a 300 KB budget. Below roughly 60 dpi, letters like c, e and o start to blur together, and at that point no further compression can save it: the information is gone.

Compress to target size finds the highest resolution that fits a KB or MB figure you type in, searching between 30 and 200 dpi before it starts reducing JPG quality. Type a number a little under your real limit — 95 KB for a 100 KB cap — since some systems measure a kilobyte as 1,000 bytes and others as 1,024, and a file that is "100 KB" by one count can be "98 KB" by the other.

Step 4: Check it the way the reviewer will

Zoom to 100% and 150% on the smallest text in the file. At a 100 KB budget, the first things to go blurry are usually small print, stamps and signatures — check those specifically, not just the headline text.

When it genuinely can't be done

A five-page document with dense small print will not be legible in 100 KB; the per-page budget is too low for the resolution that text requires. At that point, compressing harder only makes it worse, not smaller in a useful way. The honest options are: ask whether the portal accepts more than one file, upload only the pages that are strictly required, or contact the office and ask whether the limit is firm. Most offices would rather receive a legible two-page extract with a note than an unreadable five-page file that technically fits.

If the PDF was exported from Word or similar software rather than scanned, its text is stored as text and is already small — a weight problem in that case usually comes from one or two embedded images. High-quality compression resamples those images to about 150 dpi while leaving the text layer untouched, which is often enough on its own without ever going through an image-based tool.

Quick checklist

1. Cut every page that isn't required — the per-page budget is the real constraint.
2. Rescan in greyscale if the content allows it.
3. Target roughly 60–90 dpi, and type a number safely under the stated limit.
4. Check stamps, signatures and small print at 100% zoom, not just body text.
5. If it still doesn't fit, split rather than keep compressing.