Merging pdf files in Big Sur is the easiest it’s ever been. You just need to be in thumbnail view and drag one pdf fiel into the other one, save, and they will merge.

1. Double click any pdf file. (It will open on Preview)
2. Select View Thumbnails in the sidebar.
3. Drag any other pdf file into the sidebar and let it go there.
4. Save the new pdf file.

Here’s how to do it with pictures.