Question: How would ya'll recommend that I convert a FlowDocument to PDF to be attached to an EMail?

The FlowDocument is dynamic, not static.

I would prefer to be able to store the PDF in memory as a byte[], rather than on disk, but that is negotiable.

Thanks for your help!

Answer: 
Disclaimer: I am writer of XamlToPDF Library, however it is free for any type of use.

<http://xamltopdf.codeplex.com/>

Its very easy to create PDF, it supports Tables and images as well.
Another option to look at is [FlowDocumentConverter](https://github.com/marcpiechura/FlowDocumentConverter). This is available in a [NuGet package](https://www.nuget.org/packages/FlowDocumentConverter/), but per [documentation](https://github.com/marcpiechura/FlowDocumentConverter) does have a 10 page limitation.