To reproduce:

On a button click call the following method:

newPdfViewer.Print(True)

When the dialog opens click the Print button, without interacting with another component. You will notice that there will be no effect from your click. Click second time, you can now click.

Resolution: 

This is not with RadPdfView issue since the dialog is Windows dialog

For more information please refer to:

http://social.msdn.microsoft.com/Forums/windows/en-US/681a50b4-4ae3-407a-a747-87fb3eb427fd/first-mouse-click-after-showdialog-hits-the-parent-form?forum=winforms

http://stackoverflow.com/questions/2648977/c-sharp-windows-forms-print-dialog-box-click-ok-twice-to-respond