On the official website :
Poppler is a PDF rendering library based on xpdf-3.0 code.
So, I think the differences may not be so huge.
Integrating popplar can be complicated, though, since it does not provide an interface for the Android system.
I am not sure about xpdf.
EDIT:
Am I having problems if I create an Android app with calls to poppler-qt4 methods?
Since the two GUI systems are very different from each other and do not have much in common, yes, I think it will be a difficult task.
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