Whenever I try to change the format of a word or phrase in a document, the
entire document changes to the new format not just the selected text.

For example, suppose I highlight a single word and click the Bold icon, all
the text in the document is changed to Bold text. I can then hit Ctrl-Z and
it undoes all the text change except for the word that was highlighted.

is this normal? How do I change just a word or phrase without having to
undo the change?