You have the ability to load a new PDF into the same instance of PDFviewer. This example
will demonstrate that for you. Notice the Folder icon in the toolbar, select that and you can choose a pdf file. A pdf can be hard coded in, but you can change the current document anytime you like.
To see the code that performs this, toggle the "Example/Code" button at the top right of the example window. Look to the left inside the pane, you can see the code in the .cs file as well.
I hope this helps you. Good Luck,