Why is one PDF 200 KB and another 40 MB? And how does “compress” shrink a file so much? Understanding what’s actually inside a PDF makes it much easier to get a small file without wrecking its quality.
Almost everyone has hit the wall: you try to email a PDF and it bounces because it’s too big, or an upload form rejects it for exceeding a size limit. “Compress PDF” is the obvious fix — but it’s worth knowing what compression is really doing, because the right setting depends entirely on what your document contains.
What actually takes up space in a PDF
A PDF is a container. The size of that container is dominated by whatever heavy content it holds, and for most large PDFs the answer is images. Consider two very different documents:
- A 20-page contract that is pure text might be only a few hundred kilobytes. Text is stored as characters plus font information — extremely compact.
- A 20-page scanned document is really 20 high-resolution photographs saved as a PDF. That can easily be tens of megabytes, because each page is a full image.
This is the single most important thing to understand: text-based PDFs are already small; image-heavy and scanned PDFs are what balloon in size. Compression targets those images.
How compression shrinks the file
PDF compressors reduce size using a few techniques, but the big lever is almost always the images. A compressor will typically:
- Re-encode images at a lower quality. Photos are usually stored as JPEGs. JPEG is a “lossy” format — it can throw away visual detail that the human eye barely notices in exchange for a much smaller file. A compressor re-saves each image at a lower quality level, and that’s where most of the savings come from.
- Downsample resolution. A scan captured at 600 DPI has far more pixels than a screen or a printer needs. Reducing it to, say, 150 DPI can cut the data dramatically while still looking fine on screen.
- Clean up redundant data. Duplicate fonts, unused objects, and inefficiently stored streams can be tidied up for a smaller (and lossless) saving.
The trade-off you can’t escape
Here’s the honest part: for image-heavy PDFs, meaningful compression is a trade-off between file size and visual quality. Push compression hard and the file gets tiny, but text in scans can start to look soft and photos can show blocky artefacts. Use a gentle setting and quality stays high, but the file shrinks less.
That’s why good tools offer levels — High, Medium, Low. The right choice depends on the destination:
- Email or web upload, viewed on screen: you can usually compress aggressively. Nobody inspects a screen document at 600 DPI.
- Something that will be printed: stay gentle. Print needs the detail.
- Already a text PDF: don’t expect much — there’s little image data to squeeze, and forcing compression can even rasterise crisp text into a fuzzier image. If a text PDF is somehow huge, the culprit is usually an embedded scan or an oversized font set.
A note on privacy
Compressing a PDF often means the document is something you care about — a signed contract, an application, a financial record. The catch with most online compressors is that they upload your file to a server to process it. For anything sensitive, that’s the opposite of what you want.
Compression can happen entirely in your browser instead. The Moriaki PDF compressor re-encodes the images on your own device and hands back a smaller file without ever uploading the original — so you get the smaller size without handing your document to anyone.
The takeaway: check whether your big PDF is really an image or scan (it usually is), start with a Medium setting, and step up the compression only if you still need it smaller. Match the quality to where the file is going, and you’ll rarely notice the difference.