I have a PDF in Russian. It displays alright and is selectable and searchable, but the text is in Russian ASCII. As a result, when I copy or search the text, I get characters like àèçéîö, which means that the encoding is correctly recognized as ASCII but the Central European code page is applied instead of the Russian one. Is there a way to modify the file to tell PDF viewers which codepage to use?
In short, no. The fonts and/or font metadata in your PDF are missing the mapping from PDF character codes to unicode codepoints. For a Latin text, this is often just a built-in encoding. For other alphabets, an explicit
/ToUnicode entry is provided. Your file has neither.
Of course, this mapping is not required to pick glyphs from these fonts, and so the file displays fine. This is why PDF files, at least in earlier versions of the standard, were not required to have such a mapping.
Short of rasterizing and OCRing the document using a program which produces modern PDFs, I think you are stuck.