Could you please tell me which adobe application you are using and the exact dot version of the application?
Could you please tell me you want to save the magnification on a page or the full screen mode?
Saving the magnification and the full screen mode are two different things.
If you want to save the pdf in full screen mode so that the other user see's the pdf in the full screen mode then you can follow the below step:
Select CTRL+D to go to the document properties, then go to "initial view" tab and check the check-box that says "open in full screen mode" and save the file.
Also If you want to save a particular magnification level for the PDF you need to select CTRL+D to go to the document properties, then go to "initial view" tab and under "Layout and Magnification" label you will find an option magnification.
You can select the magnification level which suits you and save the file.
Please let us know if this resolves your issue.
I am done exactly what you described above. The PDF will open to that magnification but when I go to a second page that Magnification changes
The goal is for all pages to be the same magnification