Oh! Well bluebeam has an outlook, word, and excel plugin that makes your MS office experience that much more annoying (just like adobe).
Overall BB is nice and once you get the hang of the keyboard shortcuts you can really generate red-lines quickly. It has a very nice/clunky markup manager which can be useful. BB's interface itself is garbage but it's better then Adobe's -i.e. Bluebeam's interface is straight from the 90's and full of wasted screen space but Adobe's tools take far too many mouse clicks to get at. Overall, I like BB better then Adobe.
Foxit is also a nice PDF viewer. You can do most of the same things in Foxit as you can in Bluebeam for free and the interface is a lot like MS Word.
Sumatra PDF is my absolute favorite stand-alone viewer but it does not allow you to markup.
Also, if your looking to just view PDFs and not markup, Mozilla and Chrome now have built in PDF viewers so it makes things super simple.
Not sure I fully understand the scope of your criteria --i.e. "integrate with other systems" (what does that mean?), etc.--but Ghostscript would probably allow you the greatest script-ability, in that you can just roll C programs by including the headers. That would mean, no gui necessary and you wouldn't have to "open a pdf" to do what you want, thus save yourself the time required to load up separate PDFs each time you want to run a script (or preform an action).
If you don't know C, and would be happy with the limited features, you can try the Ghostscript .NET wrapper.
https://archive.codeplex.com/?p=ghostscriptnet
Awesome! I had not even considered ghostscript. ill be sure to look into it. I've done C years ago so it might not take me much to past the hello world stage.
As far as "integrate with other systems," im still just stabbing a little bit the dark. Basically I wanting to improve processes through automation I need something flexible enough for me to allow automate things and where possible and if there is a way interface with other programs.