when i try to copy any text from pdf (russian lang.) i get this: Ë.Â.Êðàñíîâ (it is not russian). but it must looks like this: Л.В.Краснов. in other pdf files all work normal. this is url to pdf that does not copy in russian: http://nuclpc1.phys.spbu.ru/nucl/pass.pdf
That means the file uses an invalid font encoding. There's not much that can be done about it, short of exporting it to images, creating a new PDF and then running OCR (text recognition) on it.