The relationship between the PDF Print Monitor window and the When printing multiple documents PDF Creator window is strange.
PDF Creator (the dialog) behaves quite normally if one creates a 1-page PDF.
However, when combining more than one page, the *PDF Creator* window "hangs around" and is visible far longer than the PDF Creator window, thus giving the appearance that Print Monitor is the "main" window, where Creator is the subsidiary window.
In this situation, one can then start to fiddle with the menu or icon bar (toolbar) on Print Monitor to combine files, creating quite a complicated PDF, and then hit the Print to Save icon.
The Creator window now comes up. If one were to decide, now, that some more work is needed, e.g. another document is to be added, it would be natural to hit the "Cancel" button to dismiss the Creator dialog. Here's where the odd behavior comes in. This does not dismiss a dialog, but rather, it exits out of the program. (Ditto for the red X button, though this is a more obvious killing of the program.)
1. Don't let a single button-click kill the program if more than one print has been done, or if any combining action has been undertaken; rather, confirm that the user wants to kill the program.
2. Don't let any "Cancel" button kill the program, but rather use "Quit" or "Exit" instead. The default for Windows is for "Cancel" to close dialog boxes, not stop program execution.
3. Consider making the Print Monitor window a subsidiary part (a pane) of the PDFCreator window, something like Windows Explorer's Folder and Files panes, or the way some Windows dialogs have a "More Detail >>>" button that provides the additional information.
It would be nice to be able to provide filename information to PDFCreator in some way other than the name of the file at the time it is "printed" to PDFCreator, which may be a nondescript "Document1". Office programs from verious manufacturers allow for document properties of various sorts. These could be searched for various names, or for a name that PDFCreator would specifically demand, and this field could then be filled in advance, say, by a macro, so that the actual filename etc. is not modified.
Another nice feature would be to be able to place (an) identifier(s) on the various subdocuments being combined into one. E.g. the first subdocument may get no identifier, the second might be "Fig. 1" or "Exhibit A", the third might be "Fig. 2" or "Exhibit B", etc. ... especially if a page number could be added in addition to this, where for example, Fig. 2 may consist of 3 pages, so they would be Fig. 2a, Fig. 2b, Fig. 2c (the "a, b, c" method of numbering pages would have been chosen). The idea would be to "stamp" the pages of the final PDF with these subpart/page numbers as the coherent whole is being created, with some reasonable selection of "stamp" locations, fonts, sizes, rotation, etc.