An EPUB file is, structurally, a ZIP archive holding a set of XHTML files (one per chapter), a CSS stylesheet, an OPF manifest describing the book's metadata and reading order, and often a folder of embedded images. It's built specifically for reflowable reading — text resizes and re-wraps to fit whatever screen or font size the reader app is using, which is exactly what makes an EPUB pleasant to read on a phone, a tablet, or a dedicated e-reader. PDF is the opposite philosophy: every page is a fixed, unchanging canvas with content locked to exact coordinates, which is precisely why PDF is the format everyone reaches for when a document needs to look identical on any device, print cleanly on paper, or be archived in a form that won't drift as software changes.
This conversion runs directly in your browser rather than requiring you to install an ebook reader or Word processor. It unzips the EPUB, walks the OPF spine to determine the correct chapter sequence (which is not always the same as alphabetical file order — EPUBs frequently name chapters "ch01.xhtml", "ch02.xhtml" out of sequence with front matter or interstitial pages), pulls the readable text out of each XHTML chapter, and lays it out fresh onto standard PDF pages with proper word-wrapping and paragraph spacing.
People reach for this conversion less often than the reverse (PDF to EPUB) but the use cases are concrete: printing an ebook on paper, sending a manuscript to someone without an e-reader app installed, or archiving a document in a format that will still open identically in twenty years regardless of what ebook software exists by then.
Going the other way is also possible with PDF to EPUB, and once you have the PDF, Compress PDF keeps it small enough to email.
