If you only need visible feedback
Look for text notes, drawing tools, highlights, Undo and a clear download path. Check whether your marks can be moved before export and whether the original document remains intact.
If the source must change
Check for true editing of existing text, OCR for scans or page-level conversion. CleanEdit PDF edits existing text and, in the web editor, recognizes scans so they can be searched. It converts PDFs only to Word, in the web editor, and does not edit recognized scans; for those, a full PDF editor is the correct category.
If the file is sensitive
Check where the PDF is processed and whether a claimed redaction actually removes content. A visual cover is not enough. For a formal signature, check the recipient’s digital-signature requirements.