There’s a free Windows utility called BeCyPDFMetaEdit that can help you remove passwords from PDF files* but without make any other change to the document. (I am not sure why the developers picked such a complex name for this useful utility).
Here’s what you need to do:

1. Launch the program and it will ask your for the location of the PDF file.

2. Before you select and open the PDF, change the mode to “Complete Rewrite.”

3. Switch to the Security tab and set the “Security System” to “No encryption.” Click the Save button and your PDF will no longer require a password to open.