PDF too big to email - how do I shrink it on Windows?

For scans the real fix is at the scanner. 600 dpi colour makes huge files; 150-200 dpi in greyscale is plenty for paperwork and comes out a fraction of the size. Rescanning is often quicker than compressing.

Depends where the PDF came from. If you made it from Word, don't 'print to PDF'. Do File > Save As > PDF and set 'Optimize for' to Minimum size (publishing online). That alone usually cuts it a lot because the images get downsampled. If it's a scan, the size is all in the images. Acrobat (the paid one) has Reduce File Size / Optimize PDF which works well. The free online compressors do the job too, but I wouldn't upload anything with bank details or ID on it. Last resort: split it into two attachments, or send it as a cloud link instead of attaching.