WPF DocumentViewer Find-function and FixedPage documents

Question

.Net contains a nice control called DocumentViewer. it also offers a subcontrol for finding text in the loaded document (that's at least what it is supposed to do).

When inserting FixedPage's objects as document source for the DocumentViewer, the find-functionality just does not find anything. Not even single letters. I haven't tried FlowDocument's yet, as the documentation for DocumentViewer is not that useful and the resources on the net are not actually existing, I now want to ask the stackoverflow community:

What does it need to get the Find-Function of the WPF DocumentViewer working with FixedPage documents?

[btw, I don't use custom ControlTemplates for DocumentViewer]