Hi cherim4,
Are you specifically trying to insert a file or page into a PDF or is this a different file type?
If you're being prompted for a password, it is possible the PDF was already password protected or a password was added to it. For example in Preview you can password protect a document. This is a completely self contained password, it wouldn't necessarily be your login password or Apple ID password.
We'd recommend confirming if you recall taking these steps when working with the document in Preview:
"Set a password for opening, printing, or changing a PDF
- In the Preview app on your Mac, open a PDF.
- Choose File > Export.
- If you want to create a password-protected copy and keep the original PDF unencrypted, enter a new name for the PDF.
- Click the Permissions button, then do any of the following:
- Set a password for opening the PDF: Select Require Password To Open Document. Enter a password, then retype it to verify.
- Set permissions: Select the checkbox next to an action to allow printing or changes without the owner password. Leave the checkbox unselected to require the owner password.
- Enter a password in the Owner Password section, then retype it to verify. It can be the same as or different from any password you created in step 3.
- Click Apply, then click Save.
Changes take effect the next time the PDF is opened."
Cheers!