How can I merge two PDF files on a Mac?

Hmmm.... that's an interesting puzzle you present because I don't know that either of the two tools you mention, Microsoft PowerPoint or Apple Keynote, support you creating a PDF version of your presentation where the first page is full size and all the subsequent pages are two or three-up (that is, there are two slides or three slides per page).

On the other hand, it's a Mac, so there are PDF capabilities running deep throughout the system, including some pretty nifty features in the PDF viewer "Preview". In fact, that's how we are going to try and solve this puzzle...

First step is to save a PDF cover sheet that's your first slide only. You can do this a couple of ways, but in PowerPoint, for example, I'll choose "Print..." then "Save as PDF..." from the lower left menu:

Notice on this same window I'm going to specify that I only want pages 1-1 printed (e.g. saved as PDF) rather than the entire document. That's done on the middle-right, where it says "Slides" and then has two boxes, one that currently has "1" and the other "25": change the latter to a "1" too and you've got just the first slide as a little PDF doc.

Now, go back to the same "Print..." dialog box and choose 2-25 for the slides to print, and on the "Print What" pop-up, instead of having "Slides" as the option, instead choose:

You'll probably want "Handouts (2 slides per page)". Remember again to specify slides 2-25, don't start with the first one, and then "Save as PDF..."

Now we have two PDF files, one that's the cover page, and one that's the bulk of the handouts. How to merge the two PDF files? Well, turns out that our friend "Preview" can do the trick, believe it or not!

Select both PDF files, then double-click to open Preview with them both included:

Notice the title of this window: "seo-page1 (1 page) (2 documents, 7 total pages)". In my case, this is showing the name and length of the title page but it's also acknowledging that I have two PDF documents with a total of 7 pages. Your should similarly indicate that it has all your slides, not just the cover page.

Now you want to choose "Select All" from the "Edit" menu, which highlights all the PDF documents in the right sidebar, then, from the "File" menu, choose "Print Selected Pages" and...

As you can see in the little preview window, it's assembled the PDF documents exactly as we hope, with a single slide cover sheet and multi-slide-per-page handout sheets and selected them all by default. You can print this, but let's actually save it as a combined PDF file instead.

Remember how to do that? Right, the PDF button on the lower left. Simply choose "Save as PDF...", give it a new name, and you're done.

Nice! Thanks, Apple, for giving the Preview app this slick capability.