Why isn't a document readable in Russian with Polaris Office Viewer 5 for Galaxy S4 mini?

[AC Question]

Document is not readable in Russian with Polaris Office Viewer 5 for Galaxy S4 mini

I have just installed a new application Polaris Viewer 5 for my Galaxy S4 mini. I have downloaded a rtf document in Russian but the text appears to be in special computing signs instead of Russian letters. What can be done? Thanks for your help.