How to pick the right tool
Start with what's wrong with the file. If a website won't accept the format, convert it: HEIC to JPG for iPhone photos, WebP to PNG for images saved from the web, JPG to PDF when a form wants a document. If the format is fine but the file is too big, compress it to the exact limit. If the picture or pages look wrong, crop, resize or organize before you compress.
For PDFs, merge separate files into one, split out the pages you need, organize to fix order and rotation, and lock or unlock for passwords. Every tool runs in your browser, so nothing is uploaded and there's no account to create.