JS can't be used to replace text in a PDF, unless that text is inside a
form field or a comment.
What you can do, though, is overlay the text you find with a text-box that
contains the new text, but it might look a bit odd...
The overlay is not rly option.
Any ideeas then how to retreive "ă" char from mysql database (where is coded as "ă") in a pdf form field?
Wait, so this text is in a form field in the PDF? If so, then it should be possible to replace it. In fact, I recently published a script that does an automatic Search and Replace operation in text form fields. Have a look here: http://try67.blogspot.com/2012/01/acrobat-search-replace-text-in-form.html
I've just tested it, and it works just fine if you enter "ă" as the search term and "ă" as the replace term.
Exactly how are you populating the form with PHP? Are you modifying the PDF directly on the server or generating in FDF that used to populate the fields?