Every time you upload a PDF or image to a random “free” compression site, you’re giving them the entire file (sometimes including hidden metadata like GPS location, device info, timestamps, and embedded text) along with your IP address and usage patterns. You have no control over how long it’s stored, logged, analyzed, or breached later. Compression doesn’t require a server; it can run entirely in your browser or offline on your machine.

If you care about privacy, avoid uploading sensitive documents to third parties. You can download & use a tool like THIS for local compressions, or if you have Linux you can use tools like Ghostscript to compress locally.