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Compress PDF files: what actually shrinks
By Alberto Gulotta · Updated · 7 min read
Nobody wants to compress pdf files. They want to get past an upload box that has just refused their document, usually minutes before a deadline, and usually on an application that matters — a job, a visa, an exam, a claim.
That is why this guide starts with the one fact that turns the whole job from guesswork into a decision: in almost every document, the pictures are the file and the text is a rounding error.
The searches split by how desperate the person is. Reduce pdf size and make pdf smaller come from people with a general problem. A pdf compressor or a pdf size reducer is what somebody types once they have decided to use a tool. Compress pdf free means they have just been shown a price. Shrink pdf and optimize pdf tend to come from people who have tried once already and been disappointed. And reduce pdf file size to an exact number is the most specific of all: somebody is staring at a form that has refused them.
Everything follows from that. A text document barely shrinks because there was nothing heavy in it. A scan shrinks enormously because it is nothing but pictures. And every slider, preset and “quality” setting in every tool is deciding the same thing: how much to degrade the images.
When a form demands an exact number
Where “100 KB” comes from, and whether you can get there
Nobody picks 100 KB. It appears in an upload box on a form — a government portal, an exam registration, a visa application, a job posting — and it is a rule written by whoever built the form, not a property of your document. Which means the real question is not how do I compress this but can this particular file reach that number and still be worth submitting?
The trap is specific and worth naming. A scanned ID crushed hard enough to hit 100 KB can come out with a serial number, a date or a signature no longer legible. It will be under the limit and the form will accept it — and then a person on the other end will reject the application, weeks later, for an unreadable document. Getting under the limit is not the goal. Getting accepted is.
For a scan there is a better lever than any compressor: go back to how it was made. Rescanning at a lower resolution, or in greyscale rather than colour, produces a small file that is still sharp — because the detail was never captured, rather than captured and then destroyed. A phone photograph of a document, retaken deliberately instead of grabbed, does the same.
What a compressor is actually allowed to throw away
The panels in Adobe’s own PDF optimiser, in its words. Read 21 August 2026.
- Images“Reduce file size by downsampling and compressing images.”
- Fonts“Reduce file size by unembedding or subsetting the fonts.”
- Transparency“lowering the quality of transparent objects and flattening transparency effects.”
- Discard Objects“Remove elements like embedded thumbnails, search indexes, and flatten form fields.”
- Discard User Data“Remove comments, forms, multimedia, and private data to decrease file size.”
- Clean Up“Remove invalid elements such as bookmarks or links.”
Two of those deserve a second look before you press the button on a document going to an authority. Flattening form fields and removing forms can turn the answers you typed into a picture of answers — or remove them. On a filled-in application that is the difference between a submission and an empty page. Compress first, fill in afterwards.
The same line has a good side. “Private data” travels inside PDFs — author names, edit history, comments left by somebody else. An optimiser run on your own machine strips a good deal of it as a side effect, which is one more reason to do this offline rather than on a site.
A note on why people ask for an exact size, because it explains what the converters sell. Search for an exact target — compress pdf to 100kb, compress pdf to 200kb, compress pdf to 500kb — and every result is a tool built for that exact number, with no explanation attached. There is nothing wrong with those tools. But a page that only presses a button cannot tell you the one thing that decides the outcome, which is whether your document can survive the journey.
Where to start
Five ways in. The second is the one with a hard number attached.
- “The upload box says the file is too big.”
- Start at the free routes
The free routes
What each system can do without an account or an upload. Only one of the three has a documented answer built in, and this wing says so.
Not covered here. It will not promise a size. Any page that says “compress any PDF to 100 KB” is either talking about a one-page scan or not telling the truth; what is achievable depends entirely on how many pages of pictures are in the file, and this guide works from that instead.
It will not rank the compressors. That verdict needs the same document run through each of them at matched settings, with the results published — a test rather than a table.
And it will not pretend compression is free of cost. Apple states plainly that the result “may be of lower quality than the original”. Every route here is a trade, and the guides say what is being traded away. What holds instead is simple: the free route on a Mac is documented by Apple and needs nothing installed.
Paper you will have to find again
Getting something off paper and into a file that still opens, is small enough to send, and counts when somebody asks for it.
- The same job, in the other places it comes up
- How to organize receipts, and how long to keep them
- How to scan a document on iPhone, and on Android
- How to sign a PDF, and whether it counts
Sources
- Apple on reducing the size of a PDF in Preview — the Quartz filter steps, the manual options and the quality warning — support.apple.com, read 21 August 2026.
- iLovePDF on how long uploaded files are kept — www.ilovepdf.com, read 21 August 2026.
- Adobe — Reduce PDF file size with advanced options, the PDF optimiser panels quoted on this page — helpx.adobe.com, read 21 August 2026.
Written by Alberto Gulotta
Founder and editor of AI Tools Primer, writing from Palermo, Italy. Thirty-five years of taking computers apart, starting with a Commodore 64 — the long version is on the about page.
Something wrong on this page? Write to aitoolsprimer@gmail.com and it gets fixed.
Written on 21 August 2026 · last checked 2 September 2026.
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