Import them to something that understands them, maybe LibreOffice Writer or some other program. Export in XML, HTML, RTF, ODT, MS-DOC or DOCX.
If not in Docx, import to Word or LibreOffice Writer (if some other format was possible), then save as in DOCX and use Calibre to convert to an ebook, ideally ePub2.
PDFs are for preview and interchange to print on paper. Ghastly for reading on eReaders even if page size is small. A5 PDFs do work on a Kobo Libra. Can be too slow and hard to read on original Kobo H2O.
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