What files can I attach, and how big can they be?

Any file type: images, PDFs, audio, archives, whatever you drop in. Every file is encrypted on your device before upload, exactly like note text, and the Files view collects all attachments in one place.

The per-file limit is 5 MB on the free plan and 50 MB on Pro. Files count against your total sync storage (50 MB free, 500 MB on Pro, expandable to 5.5 GB with storage add-ons), and the storage bar in Settings > Storage always shows where you stand.