Plain text is a simpler file type, slightly less demanding on disk storage space and memory than rich text. But most people prefer the formatting features in rich text, Unless you have a pretty slow CPU and/or very limited HD storage space, I doubt you would see significant speed improvement by converting all your rich text files to plain text.
 PDF+text denotes a PDF file that contains text “under” the PDF image. The document appearance is faithful to the original PDF document, and the text can be searched and analyzed by DT/DT Pro.
 DT uses the Index import method to capture the text of Word documents into RTF files in the database. This rich text file retains (in the Info panel) the Path to the original Word document, which is external to the database. So .doc does not equal .rtf.