Open a file you want to merge with an existing or new PDF.
Go to File > Print
Select Printer: PrimoPDF
Click Print
The Primo PDF window will open. (See below)
Click Create PDF
You will now get prompted what you want to name your PDF file and where you want to save it.
Name it the same thing as the PDF file you want to merge it with and click Save.
You will not get prompted if you want to Append, Overwrite, or Cancel
To merge the files, click Append and it will merge them together.
IMPORTANT NOTE: Don't choose overwrite or you will lose your original document.