Hope you are doing well and sorry for the trouble. As described, when you open the PDF files they open too small.
Is it the zoom level of the PDF documents that is too small? If yes, please try the following preference settings:
Go to Edit (Win), Adobe Acrobat (Mac) > Preferences > Page Display > Under, Default layout and zoom set the zoom level as per your requirement (ie: 100% or more) > Click OK and reboot the application and see if that works.
Let us know if you are referring to something else.