PDF files can be protected with a password / passphrase that will be required to open the PDF file. Enter your passphrases in the plugin settings to have the plugin automatically remove this encryption / restriction from PDF files you import.
On the right-hand side of the plugin’s customization dialog, you will see a button with an icon that looks like a green plus sign (+). Clicking this button will open a new dialog for entering a new passphrase.
Just enter your passphrase for the PDF file, then click the OK button to save the passphrase.
On the right-hand side of the plugin’s customization dialog, you will see a button with an icon that looks like a red "X". Clicking this button will delete the highlighted passphrase from the list. You will be prompted once to be sure that’s what you truly mean to do. Once gone, it’s permanently gone.
Once done entering/deleting passphrases, click Close to exit the customization dialogue. Your changes will only be saved permanently when you click OK in the main configuration dialog.