Need to combine several protected documents into one continuous multi-page document

Which type of password protection is applied to these documents? Is it forms protection applied through the Review > Protect Document button, or is it a password to enable opening the document or one to enable editing?

For documents with forms protection, you can use the Insert > Object > Text from File command while the cursor is in the destination document. The content of the form will be inserted at the cursor, and it will be unprotected. The entire destination document must be unprotected until the last of the inserts is completed; then you can protected the whole document.

For the password to open the documents, there is no substitute for unprotecting the source document; until you supply the password, the document contents are encrypted. It would be possible to write a macro that will open each document using its password (which must be stored in or available to the macro) for the second or so needed to copy/paste it, and then close it.