PDFs are often final versions, but final does not always mean perfect. Maybe a date needs a note, a typo needs covering, or a reviewer wants a visible comment on one page.
If you still have the source document, editing there is usually best. But when the original is missing or the change is only a quick annotation, a browser PDF editor is more practical.
Use Edit PDF for light edits and annotations. For heavy text rewriting or layout changes, convert the PDF to Word or go back to the source file.