Protect files with a password

Protect files with a password

I need to password protect files (with C#). files can be in any format
pdf, txt, xml, excel, word.
I found out that i could use various 3rd party libraries to password lock
PDF files, but not all the other files available out there.
Can someone please direct me with the correct approach to solve this.
In brief when an user double clicks on a file he/she should be asked to
enter the correct password to view its content.
Sorry i don't have any code to demonstrate my findings.