Issue
If you have loaded a PDF into any of the PDF reader tabs and it appears blank, the reason for this is down to the logged in user not having any write permission to the ExamWritePad folder.
Solution
Within the ExamWritePad folder is a folder called "Assemblies", inside the assemblies folder is another folder called "pdfium". Make a copy of the pdfium folder and paste it directly into the root of the ExamWritePad folder so that it sits alongside the ExamWritePad application. Now your PDFs will load correctly.