For this to work each thread would need to open a separate PDFDoc instance that refers to the same document on disk. Please note that PDFDoc does not load the entire file in memory so opening multiple PDFDoc-s is not as costly as it seems. As a starting point, please take a look at PDFDraw sample project: http://www.pdftron.com/pdfnet/samplecode.html#PDFDraw
We are also planning to release the next major version of PDFNet (i.e. V6) withing next couple of months. Among many features, this version will support multithreaded ‘read only’ access within the same document (i.e. you could render multiple pages pages from the same doc in parallel without opening multiple PDFDoc instances).