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Google's PDFium which is used in Android and ChromeOS is fast becoming the reference implementation to target: https://opensource.google.com/projects/pdfium

Is there a standalone client based on pdfium ?

pdfium has literally no documentation. but the good thing is that it is c which can be called from many languages.

"No documentation" seems a bit harsh - there's a Getting Started guide in docs/ [0] and fairly good commenting on the interfaces in public/ [1]