Strange characters in pdf

Hello,

i have a strange problem with "strange characters" (such as "à", "ö" etc). I have to edit pdf files produced with Adobe Acrobat by other people. In some cases, when i try to "copy and paste" text from this file into a "note" (within the same file), the strange characters appear scrambled, such as "Ÿ" instead of "ü", or "Ž" instead of "é".
I noticed the following:

1) on the screen, the original text looks correct, but after copying and pasting it into the note-tool (within Acrobat), it looks scrambled. If i paste it into another application (TextEdit or Word), it stays scrambled.

2) this happens not for all files, but only for some of them. note that all files come from the same source...

3) The same string of text looks and behaves absolutely fine when i open the pdf file with another application such as Preview.

4) In Acrobat, i can edit the scrambled notes to make out of an "Š" an "é" - but that is a time consuming way of doing things.

I think it has something to do with Font availability, or Unicode, or so, but a don't find the right thing to do. And i really need a solution, since editing within Acrobat is part of my money-earning work ...

Who has a useful hint ???

Thanks in advance,

thomas