What is hidden inside the files you send
A PDF carries more than its pages: an author name, the app that made it, dates, and sometimes comments or whole files attached inside. A photo can carry the time, the phone model and the place it was taken. None of it shows when you open the file.
What a PDF carries
Identifying fields hold the title, the author, and the app and dates the file was made with. The XMP packet stores much of the same again, often with an edit history. Comments, filled-in form fields, attached files and scripts can travel with the document too, even when you cannot see them on the page.
What a photo carries
Phones write details into every photo: the camera, its settings, the time and, when location is on, the GPS position where the photo was taken. A photo taken at home can give away your address.
The three steps
Cleaning does not change what is printed on the page. A name in the text, a letterhead or a visible signature stays in the file.
Then what
For names and numbers on the page itself, Redact removes the content instead of covering it. For a photo, saving a copy with Convert image writes a new file from the pixels, so the camera details are not carried over.