Mark up a PDF with highlights, comments, shapes and freehand drawing. Annotations sit in their own layer above the page, so you can comment on a document, flag a paragraph for attention or sketch on a diagram without altering the content being reviewed. They stay as annotations in the saved file, which means they can be removed again later.
Yes. They are saved as standard PDF annotations, so any mainstream reader will display them.
Yes - that is a property of the format, not an oversight. Remove PDF Annotations strips them all in one pass. To make them permanent instead, use Flatten PDF, which merges them into the page.
No, and drawing a filled black box over text is the classic redaction failure - the text is still there and can be copied straight out. Use the Redact tool, which removes it.
Files are uploaded over an encrypted connection and processed on CoonTool's own server, not in your browser. The working copy is deleted automatically once the job is done, and in any case within 24 hours. Nothing is stored permanently and no account is required.