Change PDF file size

A form wants a number, so give it a number

Some forms cap the file size and refuse the upload without telling you how far over you are. A slider marked medium is no help against that. You need the output to land under a specific count of bytes.

The compress tool takes the number directly. Type 500 in the KB box, or switch the unit to MB and type 1. The unit switch treats 1 MB as 1,024 KB, so a 1 MB target is 1,048,576 bytes.

Watch out for the other kilobyte. Some upload forms count 1 KB as a flat 1,000 bytes, which makes their cap tighter than the same number here - about 2.3% at KB sizes, about 4.6% at MB sizes. If a file that looks correct still gets rejected, the counter is the likely cause: set 97 KB instead of 100, or 976 KB instead of 1 MB, and run it again. The extra cut is invisible on the page.

The tool never aims above your file

The tool caps every target just below the size of the file you gave it. Load a 300 KB document and the highest target it accepts sits a notch under 300 KB, because compressing to a size the file already beats is a no-op that would waste your time and tell you nothing. A target typed above the file gets flagged while you type, then clamped to just under the file size when the run starts.

This also means the tool cannot make a PDF bigger. Nothing here pads a file with filler bytes to satisfy a minimum size rule. If a form demands a floor rather than a ceiling, the fix is better source material, usually a higher scan resolution, not a compression tool.

What happens when the number is out of reach

Four methods race for your target. If one of them gets under it, you get that file. If none of them does, you still get a file: the smallest candidate that passed verification, along with the size it actually reached. Nothing is silently shipped and nothing is silently given up on.

The gap tells you what to do next. A file that lands at 1.2 MB against a 1 MB target is close enough that fewer pages can close it. A file that lands at 4 MB against the same target has a different problem, and no setting on this page fixes it.

Which number to ask for

Which number to type depends on what the receiving system actually asked for.

• A hard cap. Use it exactly. Choosing 100 KB when the form allows 2 MB throws away quality for nothing.
• A guideline. Pick the largest size that still clears it, because every KB you leave unused is detail you keep on the page.
• No rule at all. Leave the box empty and the tool falls back to 500 KB - or just under your file size when the file itself is smaller. The text arms run before the raster reserve, so a text-first file can shrink without any image being re-encoded.

Then check the size the receiving system reports back, not the size your file manager shows. Counters disagree about kilobytes, so the number on the receiving side is the one that decides.

How often a given number is reachable

The documented pilot behind the parent compress page ran 19 compressions on 10 locally built documents in September 2026 at the default setting: text, scan and mixed files against targets from 20 KB to 1 MB. Exact targets landed on plain-text documents; scans and mixed files settled about 40% smaller no matter the target. That split is an observation from one small corpus on one machine, not a guarantee for your file.

No other sizes a form might name have a measured reach behind them, and any page that quotes success rates for those sizes have no published pilot behind them. What holds across targets is the direction: the tighter the number, the more it depends on your document being typed text rather than a scan - photo pages have a floor no verified pass goes under.

The general compression guide covers what is making your file heavy in the first place, which is the part that decides whether the number is reachable at all. The compress hub lists the rest.