Here is how to unlock / decrypt (SECURED) PDF files on windows. This uses the excellent qpdf application.
QPDF is a command-line tool and C++ library that performs content-preserving transformations on PDF files. “https://github.com/qpdf/qpdf
Note: This doesn’t work on PDFs that require a password to open. Just the (SECURED) ones that let you view them but stop you from printing/copying text/extracting pages/assembling them.
Setup: Get qpdf
Download the latest version of qpdf from this website: https://github.com/qpdf/qpdf/releases
There are a bunch of different versions but the one you should try first and will likely work for 99% of people is the one ending with msvc64.
The exe file is an installer and requires admin permissions and the zip file is a “portable” version that can be run without admin permissions once you extract it.
How to use qpdf
What I like to do is copy the target pdf file into bin folder in the qpdf application folder. The default location for the installer is:
C:\Program Files\qpdf 11.2.0\bin
Now you need to open a terminal / command line and navigate to the directory. I find the easiest way is to go there in windows explorer and right click in the files pane and choose “open in terminal”
Now you just need to execute the following command, replacing inputfile.pdf and outputfile.pdf as required. Windows command line does do command line autocomplete so you can type the first letter of the file name and hit tab to cycle through all of the file names that start with that letter.
qpdf.exe -decrypt inputfile.pdf outputfile.pdf
If the command works there won’t be any feedback on the command line, but “outputfile.pdf” will be created.