Ever since I downloaded Adobe Acrobat Reader, I noticed that all my PDFs open with Adobe, which I did not prefer as I liked the Preview interface.
In order to make preview the default app for opening pdf files, you can right click a PDF file in Finder/Desktop, and then press Get Info. Then, it will show some details. Open the drop down that says Open With: and set it to Preview. Then, press Change All. Now, whenever you open a PDF, it will always open in Preview.