The DBSearch is Boolean type, so if any of the indexed PDFs have the words in the expression, in any position and/or sequence (in your case the words "12 AND 523"), will get selected as result. The problem is that the number/reference you are searching "12 - 523" is not a single word. The method is more suitable for when the searched word(s) is/are unique, or you are able to define a Boolean expression that is unique to the PDF you are looking for.
Also, make sure you select the "Whole words only" option, or else these numbers may be found in the middle of other numbers and so returning false results in your specific search case.
If that type of number/reference is the one you will always use to search for these files, then it would be better to manually fill a custom metadata field with a single word unique reference related to that number (e.g. 12A523).