This is a very common question when using PDFNet for the first time. PDFNet initialize must be called before any other methods call with PDFNet.

// Use runWithCleanup instead of initialize
// PDFNet initialize is called within runWithCleanup
// runWithCleanup will also perform automatic memory management
PDFNet.runWithCleanup(callback, "Insert commercial license key here after purchase");

// Optional alternative to call initialize directly 
PDFNet.initialize("Insert commercial license key here after purchase");

The best location would be your application start-up function (such as main(), OnAppStart(), DLLMain(), etc. - depending on our application type and development platform).