No, you do not get email notification when a reply is added. You have to get back to the forum to check. You never know, someone may have posted another problem that you could help them with! Or another topic where you could join in the discussion!
I've looked into this PDF "problem" a bit more.
I have never tried drag'n'drop to add PDFs to PagePlus until checking your problem. I have just been trying to reproduce what you have found, and this is the result.
The first one opens OK. I add a blank page, and drop a second PDF onto it. As you say, I get the second PDF displayed in a single page publication, and the original PDF that was on page 1 has disappeared.
Or has it? No! If I check in the Window tab in PagePlus, I find I now have two publications. Each of the PDFs is in its own separate PagePlus publication. The blank page I added to the first one is still there, and is still blank. This happens when I try to add further PDFs by drag'n'drop as you describe, even multipage PDFs.
What you have found is perhaps a minor bug (which will never now be remedied). But if you want several PDFs in a single PPP file you will have to combine them using Insert as I described.
Geoff