What was used to create the PDF?
It was likely been created with an embedded font subset and no toUnicode mapping. Basically the codes of the characters used in the content of the PDF are mapped to glyphs embedded in the PDF which are displayed, but there is no mapping from these codes to regular Unicode codes so copying them produces gibberish. The only way to extract the original contents would be with some form of OCR.
Another possibility is that the application you are pasting it into is not shaping the characters correctly.