You could add a Form Text Field where desired and enter the text string you want for.
Find will locate this text.
A scanned image containing images of text has to undergo OCR in order to use Find / Search.
Could I make the form invisible so that it doesn't show on the page?
Invisible - sure (sort of).
When you place at form text field a 'configuration' dialog is shown if you select and display "Properties".
You can select Border Color, Fill Color, and Text Color.
If the scanned image's background color is actually "white" a choice of:
Border Color set to No Color.
Fill Color set to No Color.
Text Color set to White.
However, a background color for the scanned image that shifts away from "white" will result in something less then "invisible".
If such is the case you have to accept the compromise - perhaps something visible but 'keywords' available.
Note - I'm assuming the scan was B/W rather than grayscale. If grayscale then more of 'something' visible.
Thanks for your help! Your idea sounds pretty easy and straightfoward. I will give this a try, although most of my documents are not on white background but it will be for interal use anyway. Thanks again!