@off_grid_living@twtxt.net is it locked because of a DRM thing or something else?

Otherwise you can check if you already have the pdftotext command that comes with the poppler-utils package, try converting converting the pdf into a text file and copy to your heart’s content. I have just tried it myself.

If you don’t have it already here’s what you can do on Ubuntu or any Debian based distribution of Linux:

• Update and upgrade your packages:
  > sudo apt update && sudo apt upgrade
  
• Install the poppler-utils package
  > sudo apt install poppler-utils
  
• Now you can convert your pdf to txt file with:
  > pdftotxt -layout -enc UTF-8 name_of_source_file.pdf name_of_destination_file.txt
  

You can always do a pdftotxt --help to see the rest of possible options.
Hope this helps.