thanks for the post and welcome to the Tracker Forums.
I think you should look at the PDF-Tools SDK. http://tracker-software.com/product/pdf-tools-sdk
Assuming your PDFs are being created as text based PDFs and not image based then you should be able to search the strings directly on the PDF without the txt file in between.
If it's image based then you'd need to OCR the PDF first then search the text.
All the SDKs are fully functional, even in 'Trial Mode' and you can test every aspect of your program before committing to a purchase. The caveat is that until licensed anything you do with the SDK will result in water marked PDFs. Once you are happy that you have the right solution simply purchase a license, inject the serial keys and dev code we give you into your source code, recompile and go...
I hope that helps. Do be sure to let us know if you have further questions.