First, you need valid viewer license.
PdfPrintingNet library has 3 different licensing types: Viewer, Print and Full.
If your license type is Viewer or Full than you have valid Viewer license and you have received correct license key in email when you purchase PdfPrintingNet library.
In your source code, you will have something like this:
var pdfViewer = new PdfViewer();
pdfViewer.SetLicenseInfo("your company name", "your license key");