Hi Scott,
I am not sure if I understand your requirements right. If you need to create a pdf file using PDFLogicdocument[flow model] and then use it in grid model without saving to disk it is possible, you can do that by saving the document to memory stream and then load the pdf file from memory stream using PDFDocument.
Here is a sample for your reference for doing this:
Sample.zip
PDFDocument has many functionalities to merge, append, split, load a page from a pdf document in memory stream. Please take a look and let me know if you any questions.
Thanks,
Ajish.