Nowadays EPUB is a common document format since it's well-suited for display on mobile devices. In order to use EPUBs with PDFSpeech, these documents have to be converted to PDFs. There are many OS X apps which can do this however in general I've found the quality of conversion quite poor, in terms of adequate page margins, and re-sizing of images to fit the PDF page.
In my experience, the best app for EPUB to PDF conversion is Clearview, available on the Mac App Store, priced at $10.
While it has useful settings to control line height, page margins and text zoom, a very interesting feature is that even as an EPUB reader, it actually displays the PDF equivalent of the EPUB! When you resize the window, the dimensions of the PDF page change too (similar to the behavior of the Insert Clipboard Contents window in PDFSpeech to make single-page PDFs.) Unlike other EPUB readers, in Clearview, the PDF you can export is identical to what you see on screen. Furthermore, Clearview's PDF export replicates the table of contents from the EPUB, making the PDF also easy to navigate within PDFSpeech.
Clearview for macOS is excellent for converting EPUBs into PDFs - what you see is what you get in terms of PDF output.