Macs handle PDF files natively (no extra software required). This is one more reason on my ever-growing list to switch to a Mac.  Joining two PDF files could not be easier.  When you double click a PDF file, a program called Preview opens the file.  You can choose to have thumbnails, in a sidebar, that represent each page.  You can select multiple thumbnails and drag and drop them into another PDF file -in another Preview window to join them!  Save teh file as a different name and you now have joined the two PDF files into one document!

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