It's best to do a conversion.
For plain text files, if you have a carriage return at the end of every line, you'll see all the line breaks in Reader, and it won't look very good. The default text size is, for me at least, way too small. If you look in the "Reader Content" section of this forum you'll find a wonderful Word macro posted by Stingo which does a beautiful job converting text files into nicely formatted RTF for the Reader.
The problem with PDF files is that they are virtually always formatted for US Letter or A4 page size and hence they appear too small to read comfortably on the Reader. If you create a PDF specifically for the Reader, with the correct page size, that's fine, but you won't, I think, have much joy with PDFs you've obtained from elsewhere.