Why PDFs get so large
A PDF’s size comes almost entirely from images. Text and simple vector graphics take very little space, but a single full-page photo or scan can add several megabytes. A 40-page scanned document made with a phone camera can easily reach 50 MB, even though it contains no more information than a 2 MB text document.
That is why compression results vary so much between files, and it is worth knowing what your PDF contains before expecting a big reduction.
What PDF Studio actually does
Compress PDF finds the JPEG images inside your document and re-saves them at about 60% quality, scaling down any image larger than 1600 pixels on its longest side. It also removes document metadata such as author and title fields and rebuilds the file so the old, larger copies of the images are not left behind inside it.
There are no sliders or presets: the same sensible settings are applied every time, and the tool leaves an image untouched when it cannot be safely re-encoded or when re-encoding would not make it smaller. Text, fonts, and vector graphics are never modified, so text stays perfectly sharp.
What to expect from the result
Scanned documents and photo-heavy PDFs usually shrink the most. PDFs that are mostly text — exported reports, contracts, e-books — are already compact and may change very little, because there is nothing to reduce. If the output is about the same size as the input, that is the tool telling you the file was already efficient, not a failure.
Images that are PNG-based, use transparency, or are in CMYK color are deliberately left alone, since converting them can change how they look. If your PDF is large because of those, the savings will be smaller.
Getting under an upload limit
If you are trying to meet an email or portal limit, compress first and check the size. If it is still too big, you can split the document into parts with Split PDF, or remove unneeded pages with Delete Pages. For scans, scanning at a lower resolution in the first place (150–200 DPI is plenty for reading) is often the most effective fix.
Compression runs in your browser, so the document stays on your device.