I figured it out - We had to make combined pdf's for college financial aid verification and realized that on the new Mac's nothing works the same as the old OSX systems which were more straight forward.
Method 1: If all your pdf files are consecutive in the finder:
1. Click on first pdf.
2. Hold shift key and drag mouse to highlight/select all the others as a group
3. drag the group to the Preview alias in your dock
4. All the pdf's should open in a single file
5. Make sure they are in the order you want (in my case the first file was at the bottom and I dragged it to the top position)
6. Select all PDF's EXCEPT the first one as a group (Click second pdf, hold shift key, select the rest so they're all highlighted). Drag the entire group on top of the first pdf.
7. A black line will appear below the first pdf. Hit the arrow key above it (upper left) to uncollapse the list and you'll see all the duplicates of the pdf's appear below it.
8. Save the file and the originals will disappear but the pdf's above the black line will save as a single, multipage file.
Alternative if your pdf's are not in the same folder one after the other:
1. Open the first pdf file you want to combine with others
2. view thumbnails.
3. delete any pages you don't want (if a multi-page document)
4. drag in each of the pdf files you want to consolidate into the thumbnail box. You can drag them straight from the Finder if you want. (I didn't open them individually)
5. Put the first file on the top. Then drag the next file on top of it. (the first pdf will be temporarily be outlined by a grey box when you do this correctly).
6. Let go. The first pdf should reflect a "1" beneath it. The second pdf may not show up. If that happens, notice there is an arrow button above the first pdf. Click to uncollapse the "tree" and the second pdf will show up. Beneath the two pdf's will be a thin black line. The original copy of the second pdf will still be listed below the line. Don't worry, when you save, that will go away.
7. Now drag all the other files, one at a time, above the black line. A duplicate will appear with the number "1" below it. Your original will still be below the line.
8. Keep going until all your pdf's are duplicated. When you save - the files above the line will be in a single document, the originals below the line will be deleted from that specific file.
(you can also highlight all the pdf's except the first one and drag them on top of the first pdf has a group)
Easy once you get a hang of it, but wish Apple provided better instructions so we don't have to find it by trial and error.
