Take a look at activity HTMLToPDF and see if any of the parameters listed there will help. Of course, in order to make use of those, you will need to call that activity more directly then it is being called now, so that you can modify the parameters.
This is a bit of a hack : but I found it to work for a simplistic test case - you can use a Java Step to 'merge' two (or more) PDFs; and (at least in my testing) each document will be appended starting on a new Page.
I (briefly - this is not 'bullet-proof' code by any means) tested this on a PRPC722 system and it worked for me.
The Java Code assumes you already have created two PDFs and have them stored on the Parameter Page (which is what HTMLTOPDF does in the parameter 'PDFDocument'.
I need this functionality for 7.1.8v, but I don't have all the library classes that you are using for merging. Attached the JARs screenshot from pr_engineclasses tables. Could you please help me to achieve this functionality.