Is it possible that, if I do a search on an entire document, the program will keep those words highlighted even after I've started a new search?
e.g. I search the entire document for the word "say". The program highlights all the "say" it found, even after I've done a search for the word "said". And so on and so forth.
There's nothing like that, but you could always do a Find All and then add the highlighting (or background color) yourself.
Removing it is a bit trickier if you already have coloring in your document, but can be done. Just apply a color that you aren't using elsewhere in your document. Then when you want to remove it:
1. Select any single highlighted bit of text.
2. Click the highlighter tag (the little square at the bottom-right of each document window).
3. From the menu choose "Select All".
4. Now remove all the selected highlight using the menu Format > Highlight > Remove Highlight Attribute.
I'm suggesting you apply a highlighting attribute to the matches (add formatting to the document), eg:
1. Open the Find panel.
2. Type your search text / expression.
3. Click the "Find All" button.
4. Move back to the document window.
5. With all matches still selected, apply the highlighting, eg: use the menu Format > Highlight > Yellow.