You can search for text while previewing the following file types on Dropbox:
To preview a file on dropbox.com, click the name of the file in your Dropbox account. You can also preview files that are shared with you by clicking on the email or link that was sent to you.
To search for text in a document file:
You’ll see the total number of matching results display in the search bar. Your current match will appear in green, and your other matches will appear in yellow. Use the arrow keys in the search bar or press enter to switch between matches.