The Keywords entry in the document properties for a PDF file is stored as a string, so Acrobat does not try and break apart your entries or reformat the delimiters. It's suggested that you use commas to separate keywords in the list, and you can use double-quotes to identify a phrase - though PDFs created by other software will often use semicolons. There are no absolute rules in the ISO PDF specification.
Interpreting the keywords string is up to the software performing the search (e.g. a Web search engine spider or document management server). Most will be equally-happy with commas or semicolons, and most will understand the idea that "a quoted string" is to be matched as a block.