There are two functions: Rotate View and Rotate Document.
Rotate view just rotates it on the screen, like you would turn a piece of paper on the desk for a better look at some element. You have not changed the document orientation and if anyone has the Rotate view controls, they can do that if needed. so Rotate view settings are not saved.
Rotate Document permanently changes the orientation of text or tables pages in the document and saves it for everyone who opens it.
It's under "Organize Pages" and you can change it in 90 increments for selected pages, for example if you wanted a page to be printed out that way. You then save the changes.
You would need Acrobat Standard or Pro to do this. I'm not sure Reader can.