The default paper size in Word, Acrobat, etc., keeps reverting back to US Letter Small. I want US Letter instead! How annoying! It actually will shrink pages to make them fit this imaginary paper size! (OK, it is a real paper size, but I have never used it).

It's fixable, but it is a secret how to do it.

Open up page setup (under the File menu in whatever program this is happening in). After making your changes in the Page Setup dialog, press the option key while clicking OK. An alert asks you to confirm whether you want to save the current Page Setup settings as the default. You have to perform this exercise separately in each application that has its own Page Setup settings. To see if a particular application has its own Page Setup settings, look for the application's name in the pop-up menu at the top left of the Page Setup dialog box (see "Personal Defaults"). If that pop-up menu doesn't list the current application's name, the application uses the generic Page Setup default settings. Changing the generic Page Setup defaults in one application affects all applications that use them (but doesn't affect applications with their own Page Setup settings).