TOPIC:

I need to cache some data (fonts for example) when I switch between pdf documents. Can I do this with your library API? What does the method Document.SetCache() doing?

no, you need to implement it your self.

SetCache, is used internally to save temp data while rendering (e.x. compressed images) and these data is deleted when the document is closed.

Is caching turned on by default? I want to be able to open a pdf entirely in memory without having cached data written to the device.

SetCache is just used for created images/fonts/and some import page data.
other words: SetCache are just for some new objects, that added to document.

you can use PDFMemStream and Document.OpenStream();
or can use Document.OpenMem();