PDF Files are sometimes delivered with write protection. You can configure okular to not obey them, or you can use the cli tool qpdf to disable DRM on the files completely.

Via a graphical user interface for single files

Install the pdf view- and editor tool “okular” and click:
Settings – Configure Okular – Obey DRM Limitations

For multiple files via a shell script

This small script disables DRM for all .pdf files in the directory “$olddir” and copys a nonprotected version to “$newdir”. You have to install the tool “gpdf” for this script.

#!/bin/bash

olddir="/home/user/protected-pdf-files/"

newdir="/home/user/writable-pdf-files/"

cd $olddir

for file in *.pdf; do
  echo "Deactivating DRM for $file"
  qpdf --decrypt "$file" $newdir/"$file"
done

exit 0