How to edit a PDF that I downloaded from a website?

Some PDFs are image-based so it's unable to edit. Since I don't have the file so I can't tell if your PDF is possible to edit. I use Foxit and PDFgear to process PDFs and PDFgear is free. So there's no threshold for you to try that. Be cautious when editing in case the layout gets messed up.

PDF stands for Portable Document Format and it's not designed for editing. It keeps consistency when viewing on different devices and slight edits might change that. If you can't get the original file, try to convert PDF to Word to make it easier for editing.