Read information inside PDF?

I'd like to automate the invoices I receive from a particular company. Unfortunately they name the files very strangely and the filename is not at all related to invoice number, or date. So I'm trying to find something to extract information from inside the PDF. I would look for particular strings and numbers. I can think of Spotlight that would be able to do that, but I don't think that's the best way. Suggestions and solutions to this particular problem are welcome.

Thank you!