A document open password is a password used to restrict the opening of a PDF file. PDF owner passwords, on the other hand, are used to provide document restrictions in PDF files.

Although this password is called the Document Open Password in Adobe Acrobat, other PDF programs may call this password the PDF User Password or the PDF Document Open Password.

Some PDF readers allow you to protect the PDF opening with a password, but it is usually specialized tools that have that option. There are also some PDF creators that have the option to create a PDF user password.

You can install the free trial version of Adobe Acrobat to password protect a PDF, or of course use the full version if you have it. Use the File > Properties menu, then the Security tab to find the Security Method option. Choose Password Protection, then in the new window select the option Require a password to open the document. Enter a password in that text field to create the Document Open password for the PDF file.