Avast claims the latest Adobe is not safe
Avast is talking utter bull. If you downloaded an official copy via the official website, it is not unsafe. End of story, no argument. Your Avast definitions are either wildly out of date or there's some bug in Avast.
Adobe are a large, reputable company. There is nothing to worry about.
Infact, you should be more worried about Sumatra which looks like its some sort of one-man-band job... not only would I be worried about potential hidden stuff, but I would also be weary of the lack of long term support. I would also suggest you look at the whole litany of unresolved bugs in Sumatra see here....
Issues - sumatrapdf - PDF, EPUB, MOBI, CHM, XPS, DjVu, CBZ and CBR viewer for Windows - Google Project Hosting ... that lot is going to take one man quite a while to fix.
o why is the Adobe pdf file a massive 138Mb download and the Sumatra file only 9 Mb?
(a) Are you sure you deselected the optional extras ? The consumer version contains optional extras.
(b) Sumatra is probably a more minimal implementation.
(c) Believe me, at some point or another you'll find certain types of document will only open in Adobe Reader and won't work with third party readers.