There are two free software tools you'll want to familiarize yourself with to convert .pdfs to .refx files.
1. The first is Calibre, and is available here. This is an all-inclusive ebook management system. You'll want to download and learn how to convert .pdf files to .rtf files. CAUTION: There are two different kind of .pdf files; for simplicity sake, they are 1) graphically oriented, and 2) textually oriented. (If it looks like a picture of a text, it probably is a picture, and is graphically oriented). The graphically oriented files will not convert to .rtf using Calibre. If you want to convert graphically oriented .pdf files, you're going to need commercial ("pay $$$") software.
2. The second is ToolTip Tool NT (colloquially called T4), which is housed here at this website and is basically a wordprocessor for Esword module making.
T4 is moderately easy to learn; just take an hour or so to read the manual and practice a little. If you're computer savy, you'll pick it up quick. If not, post help questions here at the forum.
One more comment: if you convert modules that are public domain, please remember to post them here for sharing.
My experience is that is the tricky part is getting the .pdf to .rtf; and that means learning Calibre.