I have to say
"research team has been working since early October 2018 together with experts from Germany’s Computer Emergency Response Team "
Which is after the date of my “surmise” that rumblings were floating about signature spoofing which was based on something similar to the second of the subsequently published three attack vectors. But still not sure if they cover some or all of the other potential methods that I had heard of.
Notably Adobe Reader version 9 was the most secure (most hardened over time) and of course Foxit which I had used to test shortcomings fared poorly
Inevitably SumatraPDF not listed as it cannot be compromised since it does not support scripting of signatures .