How do I reduce a PDF's file size for free?

If you made the PDF yourself from Word, Excel or PowerPoint, save it again with Minimum size (publishing online) selected, which usually beats compressing it afterwards. On a Mac, open any PDF in Preview, choose File > Export and pick Reduce File Size from the Quartz Filter menu; it shrinks image-heavy files a lot but can blur them, so check the result before you send it. Online, Adobe's free PDF compressor and iLovePDF's Compress PDF both work in the browser; Adobe deletes the file unless you sign in to save it, and iLovePDF deletes processed files within two hours. For a scan that is huge because every page is a photo, rescanning in black and white or at a lower resolution often saves more than any compressor, and a phone scanner such as Google Drive's or Adobe Scan keeps scans small to begin with. Rule: export settings first, Preview or Adobe next, and confidential files on your own computer.