The Reading Highlight is designed to highlight only while reading that session - It isn't meant to save!
If you wish to find and highlight a word throughout your document so that you can save it, then try this little macro - copy the entire code below and add it to an module in the VBA window (Alt+F11 > Insert > Module > (double-click the new Module) > Paste the code > replace the word "Quick" with "your word or string", then go to the View tab, click Macros > view and run the one called 'Find_and_highlight_Loop') - that should do the trick and save the highlight for you!
Code:
Sub Find_and_Highlight_Loop()
'
' This macro will find the word in speech marks "", select it, highlight it and
' then move on and find the next one until it reaches the end of the document.
'
' To add more words, then copy the text between the markers and paste after the
' last marker
' Don't forget to replace the word ;-)
' <---------------->
Selection.Find.ClearFormatting
With Selection.Find
.Text = "Quick"
.Replacement.Text = ""
.Forward = True
.Wrap = wdFindContinue
.Format = False
.MatchCase = False
.MatchWholeWord = False
.MatchWildcards = False
.MatchSoundsLike = False
.MatchAllWordForms = False
End With
Do While Selection.Find.Execute
Options.DefaultHighlightColorIndex = wdYellow
Selection.Range.HighlightColorIndex = wdYellow
Loop
' <---------------->
End Sub
;-)