Password-protected PDF not opening on Linux

First download version 8 of Adobe Reader in your language from the archived ftp site (version 9 does not work):

ftp://ftp.adobe.com/pub/adobe/reader/unix/8.x/8.1.7/

Then download FileOpen plugin for Linux:

http://plugin.fileopen.com/

After extracting the archive, install using commandline_installer.sh (open terminal, navigate to folder, type ./commandline_installer.sh) or copy the FileOpen.AR7.api and FileOpen.AR8.api files to the Reader plugin folder with:

sudo cp FileOpen.AR*.api /opt/Adobe/Reader8/Reader/intellinux/plug_ins/

You should hopefully then be able to open the file.