There is a solution when you are creating multiple tabs then add one web viewer within the tab and there you can mention the dictonary site.
So follow this concept and fulfill your requirment .
I wanted to know if Filemaker was able to search for a specific word/phrase in all fields across all the tabs and to highlight occurences?
You might experiment with Find/Replace in the edit menu. Find Next will at least enable you to move from highlighted found text to highlighted found text across all the fields in your current record--which is pretty close to what you describe here.
You might also find it easier to structure your database so that you have multiple records for each document instead of adding additional fields and tabs to the same record.
Oops- accidentally clicked best answer on my answer rather than PhilModJunk's post.
Find and replace does the job well enough- however, I had to amalgamate all my fields into the one tab- this makes little difference to me other than the database doesn't look as nice!
Thanks for your help!