You do not have to download additional software to read the ePub file. Rename it to .zip and extract it. There are XHTML files under the OEBPS folder. You can open them in your browser. You can also use the ePub file with any e-book reader application you might be using, be it Apple Books, Amazon Kindle, or anything else.
Beyond that, the decision to move from PDF to ePub wasn't random. Converting from the source DocBook XML 5 format of the documentation to PDF required converting it first to XSL-FO and then using Apache FOP to convert the XSL-FO intermediate file to PDF. XSL-FO was deprecated in 2013. W3G recommends that HTML with CSS3 is used instead (see https://en.wikipedia.org/wiki/XSL_Formatting_Objects#Replacement). The conversion process was also extremely slow (5 minutes for Akeeba Backup's documentation, for example) which made releases painful — especially since bugs in Apache FOP would stop the release process cold with an error that an image can't fit the page by one thousandth of a millimetre. PDF is designed as a print format file. Using it on a screen —such on a desktop, tablet, or smartphone— is problematic as it requires a lot of zooming in and out. This has been a problem reported multiple times by many users.
ePub solves all these problems. ePub is just HTML, CSS, and metadata; converting the DocBook XML source to chunked HTML is fully supported by the tooling. The conversion process is two orders of magnitude faster, and it doesn't break the release process. The content can be reflowed to any viewport dimension, making reading the documentation even on a smartphone not just possible, but actually practical.
In short, PDF was all shorts of problematic. We are not going to be producing PDF files anymore.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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