You do not need to upload sensitive PDFs to an online compressor. Preview on macOS has a built-in size reducer that works offline.
Short answer: open the PDF in Preview, use File > Export, and pick the Reduce File Size Quartz filter.
Shrinking a PDF in Preview
- double-click the PDF so it opens in Preview
- click File > Export... (not Export as PDF)
- in the save dialog, open the Quartz Filter dropdown
- select Reduce File Size
- rename the file (for example, add "-compressed") so the original stays intact
- click Save
A 50 MB scan typically drops to 2-5 MB. If the PDF contains high-resolution images, the savings are even larger.
The quality tradeoff
The default Reduce File Size filter compresses images aggressively and lowers the document's effective resolution. Tiny text and complex charts can look soft afterwards.
If the compressed output is unreadable, a dedicated PDF editor like Adobe Acrobat or PDF Expert exposes a manual quality slider so you can trade file size against clarity.
Batch compression
For a whole folder of PDFs, open them all in a single Preview window, then export each through the same Quartz filter. macOS keeps the filter selected between exports so the process is fast.
Better next routes
If other large files are clogging the drive, continue with How to Find and Delete Old DMG Files on Mac.
For the broader framing, use How to Find Large Hidden Files on Mac.