Option to add to Windows Context Menu, bringing up wimdow to add to existing PDF you choose

Basically the same way the 7Zip GUI works if you choose Add to Archive, bringing up a window or menu where you could browse to & select another PDF file. And/or I suppose you could have it compile a list of PDF files to combine, so when you right click a PDF you could add it to the list, then combine all at once when you're done.