There is no shying away from it, what follows is not entry-level geekery. Those of a nervous "what's a browser again?" disposition, look away now.
Many of us are now using digital PDFs instead of paper. I haven't done a case with physical paper since February 2020. One issue that crops up is different page sizes in Adobe Acrobat. You know the problem, when you are scrolling through several hundred pages and some are giant and others are small because they've been coalesced form different sources and documents. You spend your whole time clicking the zoom in and zoom out buttons. I am told that if you need to print the whole thing out, then the problem persists.
On a Windows machine there appears to be a way to print to file (select print PDF as the printer) but that does not work on a Mac. You must access the world of "Preflight". NOTE: Doing this will delete bookmarks you added but not ones that the combined PDFs came with. Do this BEFORE you add bookmarks. Here goes, buckle up:
In Adobe Acrobat DC, click on Tools
Search tools for “Print Production”
Open Print Production
Click Preflight in the pane on the right
Click "Prepress, colour and transparency" at the top
Of the three icons in the middle, click the spanner on the right
Find and click "pages" at the bottom of the list: that opens a long list
Click “Scale pages to A4”
Make sure “current file” and “embedded files” are ticked (bottom left)
Click the spanner that says “fix” at the bottom right
It will prompt you to save it somewhere. Use a new save name (eg “Mr X bundle scaled”)
That’s it. Next time you open it, all of the pages will be the same size.
NOTE: Doing this will get rid of all your bookmarks so do this FIRST and then bookmark.
Good luck and stay safe.