To open all PDF documents in Preview:

In the Finder, select a PDF document.

Choose File > Get Info.

Click “Open with.”

Choose Preview from the pop-up menu, and click Change All.
You can use Preview to open and print documents in Portable Document Format (PDF). A PDF file has elements of a printed document. Many PDF files are converted from printed documents or created from the same source files as printed documents.
PDF files opened in Preview can be read, resized for onscreen viewing, annotated, and printed. You can search a PDF document for specific text, navigate some PDF files using a table of contents, and select and copy text in the file.
You can set up Preview to open all your PDF documents.
In the Finder, select a PDF document.
Choose File > Get Info.
Click “Open with.”
Choose Preview from the pop-up menu, and click Change All.
If you open a PostScript (PS) or EPS file in Preview, it’s converted to a PDF file.