Originally Posted by jackhulk
... and copied kindlepdfviewer-master.zip to customupdates
This is the problem. You have downloaded the archive of the source code instead of the program itself. The program is downloadable from here:
Don't click on those stupid buttons "Download as .zip" or anything that looks like them. They are github's (very useless) way of letting users download the source code. Why useless? Because the proper way to get the source code is to clone the repository using git command.
So, instead, click on the most recent entry in the list of nightly builds. At the moment it happens to be this one:
And then do everything you did before and it will work. And if you have the old kindlepdfviewer directory it is best to remove it (to get rid of old settings etc, though "officially" this is not needed, but it is cleaner to not have any potentially useless old files lying around and confusing things).