Compress PDF
Your file never leaves this device
Most of the size of a large PDF is images. This tool decodes each embedded JPEG, scales it down if it is bigger than the target, re-encodes it at the quality you choose, then rewrites the file with object streams and no metadata. Scanned documents typically shrink by half or more; a text-only PDF saves a few per cent.
How to compress a PDF
Be clear about what this does and does not do. It re-encodes JPEG images and tidies the file structure. It does not re-run text through OCR, subset fonts, flatten forms or convert vector artwork to bitmaps, all of which desktop tools do and all of which can lose information. That means a text-heavy report will barely shrink, and the honest answer there is that it is already small.
Questions
Because it is mostly text and vectors, which are already compressed efficiently. There is no image data to re-encode, so only the structural saving of a few per cent is available.
At quality 70 and 1600 pixels on the longest side, a scanned page stays readable on screen and in print at normal sizes. Push the sliders up if you need archival quality.
No. An encrypted PDF has to be unlocked first, which is something only you can do.
No. The whole thing happens in this page. The parser is downloaded to your browser when you pick a file. The file itself goes nowhere.
Because the PDF parsing library downloads the first time you choose a file. After that it is cached and the tool responds immediately.