The most obvious difference between Calibre and InDesign is the price, and Calibre certainly wins in this area. InDesign remains dominant in design, naturally, but when saving in eBook form can't convert to all formats, and has some coding quirks that can cause the outputted …
FoxitPhantom PDF is not a direct 1 for 1 comparison to Calibre, but I called it out because some of what we'd use FoxitPhanton for is supplanted by Calibre. Calibre has built in meta-data editing tools, and that applies to PDFs as well, which is a huge benefit as we don't need …
Calibre, like Acrobat, offers the user the ability to be able to view a digital ebook whether from online or off-line. But Acrobat can only open PDF files. Calibre can open & convert multiple digital file formats.