Can I do a checkpoint on a text string in a pdf?

Sure ... if you don't want to use the Apache PDFBox - generally you can use a file checkpoint after saving the pdf somewhere - if the pdf displays online you could do an image compare - if the pdf displays online you could use OCR.  Image compare checkpoint isn't perfect - file compare checkpoint isn't perfect - ocr logic isn't perfect.  All have issues you might have to work thru - for example if you each time you produce the pdf - does the pdf contain a current date that you have to account for.....  you need to determine which option works best in your environment.