How do I Merge PDFs

In the Application Preview >> Help and type " Add Pages"

The PDF User Guide will appear with instruction on both Adding and Removing page

Excerpt below

Add, delete, or move PDF pages in Preview on Mac

You can open a PDF in Preview, then add, delete, and rearrange pages, or move pages from one PDF to another.

Open Preview for me

1. In the Preview app on your Mac, open the PDF you want to change.
2. Do any of the following:

• Add a page to a PDF: Select the page you want to appear before the new page, choose Edit > Insert, then choose Page from File or Blank Page.
• The new page is inserted after the current page.
  • Tip: To add new pages, you can also choose View > Thumbnails, click the Add button at the bottom of the sidebar, then choose Insert from File or Insert Blank Page.
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Simpler still, open a Finder window and select (with cmd key) each of the PDFs in the order you want them merged, and then, right-click on the first selection and from the secondary Finder menu, choose Quick Action > Create PDF. A new PDF will be created in the same location as the first selected PDF, using its basename with a 1 appended.