Q: The steps look straight forward, get the document, get the page, get the thumbnail:
PDFDoc doc = mPDFView.getDoc();
Page page = doc.getPage(1);
int imageInfo = page.getThumbInfo();
The doc and page object are valid, but getThumbInfo() is returning null.
A: The getThumbInfo() method does not create a thumbnail, it returns the embedded thumbnail associated to the page. Your are getting a null array because probably your document does not have embedded thumbs. If you wish to generate a thumbnail, your best bet would be to use an instance of PDFDraw to rasterize the page at a small resolution.
If you want more information on PDFDraw, you can search our knowledge base: https://groups.google.com/forum/#!searchin/pdfnet-sdk/pdfdraw or just take a look at the sample app (http://www.pdftron.com/pdfnet/samplecode.html#PDFDraw).