Compressing a PDF and compressing a PDF to a size are different jobs. The first asks "how much smaller can this get" and any saving counts as success. The second starts from a number someone else chose, and success is binary: 499 KB passes, 501 KB is rejected with the same red error as 5 MB.
Where the numbers come from
Email is the generous end — 20 to 25 MB depending on the provider. Upload forms are meaner: 2 MB and 1 MB ceilings are everywhere, and the application portals of governments, universities and employers regularly demand 500 KB, 200 KB or 100 KB per document.
The limits look arbitrary because they are — a storage decision made years ago, outliving its reason. But they are not negotiable from your side of the form, which makes them the requirement, and the file the variable.
Why the slider cannot answer
Quality is an instruction to the encoder. Size is a consequence of it. The same quality setting produces a different size for every document, because what a page costs to encode depends on what is on it — a page of typed text and a page of photographs are different animals at the same setting.
So the size is only known after encoding, and the workflow everyone knows follows from that: export, check, nudge the slider, export again. Three or four rounds of guessing at a number the tool could have been told directly.
A tool that accepts the limit itself runs that loop for you — encode, measure, adjust, again — until the result lands just under the number, or until it can say with certainty that the number is out of reach. The loop is the same one you would run by hand. The difference is that it converges in seconds and does not round up out of optimism.
The order that reaches the target
If you are adjusting by hand, the settings are not equal, and the order matters because each one spends something different.
Greyscale first, if the document is black ink on white paper. A colour scan of a monochrome page stores three copies of nearly the same data; greyscale is a third of the size and costs a page like that nothing visible at all.
Resolution second. 150 dpi is legible on any screen and is a quarter of the pixels of the scanner's default 300. This is the largest single saving and the one people skip, because lowering a number labelled "quality" feels safer than lowering one labelled "resolution". It is not. Resolution the reader cannot see is free; quality is not.
Quality last, and gently. Text has hard edges, and pushing the quality down far gives every letter a grey halo. A scan of prose stops being pleasant to read well before it stops being small enough.
Whether any of this has room to work depends on what the file is — a scan and a typed document carry their weight in completely different places, and a typed one has usually paid its dues already.
When the target is impossible
Every document has a floor, and no setting reaches below it. A typed report is near its floor from the day it was exported — usually already under any sane limit, but with single-digit percentages available if it is not. A forty-page scan compresses a long way and still cannot become 100 KB while remaining forty readable pages. Vector drawings — maps, plans — are thousands of meaningful paths, none of them removable by an encoder.
A tool that searches for the target properly will tell you when it cannot be reached. That answer feels like failure and is actually information: the file, at this page count and this content, does not fit under this limit. The move is no longer compression.
Split the document and upload it in parts, if the form allows several files. Send only the pages the recipient actually needs — the signed page, not the whole contract. If it is your scan, rescan in greyscale at 150 dpi instead of squeezing artefacts into the existing images. And when the channel permits a link, a link wins the argument entirely.
The shortcut that ruins the file
One kind of tool hits any target on any file, and it is the one to avoid: it rasterises, rendering every page to a picture and rebuilding the document from the pictures at whatever resolution fits the budget. On a typed document this destroys selection, search and screen-reader access — everything that made it a document rather than a photograph of one.
The test takes ten seconds: run the tool on a typed file and try to select a word in the result. If you cannot, the target was reached by demolition, and a form that accepts the upload may still belong to someone who needs to search the text on the other side.
The short version
The limit is a number, so give it to something that works in numbers: a size-targeted compressor that encodes, measures and adjusts until it lands under, or says honestly that nothing will. By hand, the order is greyscale, then resolution, then quality — and when the floor is above the target, stop compressing and start splitting.