Nikolay,
I've got some good news and some bad news.
Find is easily done thru Edit>Find and include your character or search string.
Replace is not so easy. Depending upon your version of Acrobat, you have to use the Touch-up Text Tool to replace your character or word with new text. You are limited to the existing space. In other words, you can't replace a word with a sentence. If you have to make large changes, you are wiser to revert back to your original document in its original format (Word, PageMaker, Publisher, etc.) make your changes, corrections, substitutions, deletions and then re-distill your PDF document.
If you are using Acrobat 6, then I may be incorrect in my assessment. I'm still at version 5.05 and there are differences between the program versions of which I'm not fully aware.
Knock and the door shall be opened unto you. Seek and ye shall find!
I love Mondays cause they're the Second day of the week!