In OS X Apple have again changed the way that 2 pdf files merge in the free OSX Preview App. To Merge 2 pdf files in OS X Mojave you need to drag the thumbnails of one pdf file into the thumbnail view of the second pdf file. You can’t drag a pdf file directly from the desktop into the new thumbnail window or the files will not merge.
Step 1. Open the first pdf file in Preview and select ‘View Thumbnails’
Step 2. Open the second pdf file in Preview and select ‘View Thumbnails’
Step 3. Drag the thumbnail that you want to merge from one pdf to the other.
4. Save the new merged pdf.
Note: Do not drag a pdf file directly into another pdf file, it will not work.
You can use the menu item ‘Edit – Insert – Page From File’ to insert an entire pdf file.