Parth, one problem with your approach is he will submit PDF and therefore you won't be able to put the PDF in a variable that's suppose to contain just xml.
The prepopulation should be the same. If you start off with an xdp, then you will call a render service that merges data with your xdp to create a PDF.
Now when you submit, you will submit the entire PDF back in the Document Form variable. In Workbench, you can use the FormDataIntegration service to extract data from that PDF that's being stored under Document Form var/object/document and put it in an xml variable. Then you can just use xPath to do your condition.
I'm assuming you'll just pass that same Document Form variable to the next step, because if you do any change to the PDF it'll brake the signature.
Let me know if I missed anything.
Jasmin