@Magnum:
It's simple why only the Gospels and the Acts are in PDF—only these first books were made available in print. You can't have a PDF of something on JW:org that has not been prepared for printing.
The Study Bible is a veeeeery large and time-consuming project. Initially when the study edition of Matthew was released at the 2015, the Study Bible only contained the full text of Matthew. Later, it was decided that the Study Bible be the default translation used for hyperlinks of Scriptural citations on the English JW.org, so in 2017, the full 2013 New World Translation was included in the Study Bible, even if, by that time, only a couple of other books had their study notes released. As of today, the Study Bible has extended features up to the book of 2 Corinthians, that was released in December with a 2020 copyright.
As soon as there is enough material to format the Study Bible into a printable format, new volumes will be released in both print and PDF. My personal hunch is that volume two will comprise Romans through Colossians, volume three 1 Thessalonians through Hebrews and volume four James through Revelation.