A lot depends on whether it is a scanned image type of PDF or a PDF with text. The former never seems to convert well at all in my limited experience. The latter tends to convert better, but I've yet to get one to be very readable, especially using calibre. I usually get garbage out. I tried about 10 PDFs of different types then gave up. Let's face it, PDFs were never intended for reading. They were intended to be printed at a specific size. Some 15+ years later Adobe has added some features and improvements, but PDFs are still difficult to view even on a PC with a large monitor. Without reflowable text, they will be garbage on readers with 6" screens, and barely tolerable on readers with 10" screens.