Rename using invoice number

This is my 1st post so forgive me if this is a stupid question.

I get invoices from a business as PDF files with the same file name of "Esale.pdf". I want to rename these, picking up the actual invoice number inside the file. The files are text, so no need to OCR and contain "Invoice No.: 000xxxxx" where the x's are the invoice number, which I would like to use when renaming the file.

Can you either advise how to do this, or provide a URL where this might be described already? I couldn't find the answer in the manual.

Thanks in advance,
Paul

Look up "match patterns" in the manual.