Here is a sample VB.NET code to highlight the marked text in PDF:
Private Sub TextMarkButton1_Click()
filey = "c:\test.pdf"
Set AcroExchApp = CreateObject("AcroExch.App")
Set AcroExchAVDoc = CreateObject("AcroExch.AVDoc")
AcroExchAVDoc.Open filey, ""
Set AcroExchPDDoc = AcroExchAVDoc.GetPDDoc
sustext = "Release"
Set jso = AcroExchPDDoc.GetJSObject
AcroExchApp.Show
nCount = 0
If Not jso Is Nothing Then
nPages = jso.numpages
For i = 0 To nPages - 1
nWords = jso.getPageNumWords(i)
For j = 0 To nWords - 1
word = Trim(CStr(jso.getPageNthWord(i, j)))
If word <> "" Then
result = StrComp(word, sustext, vbTextCompare)
If result = 0 Then nCount = nCount + 1
End If
Next j
Next i
End If
MsgBox nCount
Set jso = Nothing
Set AcroExchAVDoc = Nothing
Set AcroExchApp = Nothing
End Sub