Calibre can be used convert PDF to ePub, but when I tried doing this with the User Guide the results were far from perfect! Nevertheless, it might be worth using it to convert one of your files to ePub, for test purposes. You might get better results with a text only file, rather than one containing images.
You could try using the advance search option in this forum to search for other threads related to this topic.
Use the Search drop-down in the blue bar, instead of the Search box in the top right-hand corner.
For example, searching for 650 and PDF in the title, I found this:
I can't think of anything else to suggest, but hopefully someone with expert knowledge will chime in soon enough.