What would be the point since Acrobat is intended as a final format? Spell-checking should be done in the authoring application.
Here's what I am trying to do. I have a form that I created using Adobe Livecycle Designer. The end user will fill in the form fields in the dynamic PDF form. One of the form fields allows users to type in free text such as a description etc. As they are typing in mixed upper-lower case sentences the spell-checker highlights any misspelled words. If the end user types everything in CAPS the spell checker will not work. Microsoft Word has the ability/option to spell check all caps. Acrobat apparently does not.
This is functioning as designed. Commonly things written in ALL CAPS are company initials or acronyms, etc. In Word, you actually have to turn on the all caps spell check (Word Options > Proofing > Ignore words in UPPERCASE) as it's designed to let the same content go too.