Many public domain documents are in pdf format, and must be converted to docx before making a Logos PBB resource.
I have found that the AVS document converter does this pretty well, and it is pretty cheap. It will convert between pdf, epub, docx, html and other formats. So you can download a pdf or epub public domain book, and convert to docx in one step.
I have no interest in promoting AVS or any other software, but I found this helpful and thought I would mention it. Works for me.
Of course once the document is converted you have some work to do in preparing it - depending on the type of resource.
Again, AVS Document Converter is not free, but it is very reasonable. I bought it some time back because it has lots of uses.