PDF: error message „Document is encrypted”

When you try to save PDF you get the error message “Document is encrypted”.

Run below command in the terminal as root user or use before command sudo command:
1) Get information about new packages
robert@linux~$ sudo apt-get update

2) Install gpdf package
robert@linux~$ sudo apt-get install qpdf

3) input.pdf (source file), output.pdf (output file after decrypt pdf)
robert@linux~$ qpdf –decrypt input.pdf output.pdf

4) Open output file, makes changes in file and try to save as new file