'// tb is a textbox with search phrases per line
'// PdfViewer1 is the PDFViewer control

If PdfViewer1.Document IsNot Nothing Then

    PdfViewer1.RemoveHighlightFromText()
    Dim intPagesCtr = PdfViewer1.Document.Pages.Count
    Dim currPage As PdfPage

    For i = 0 To intPagesCtr - 1
        currPage = PdfViewer1.Document.Pages(i)

        Dim parts As String() = tb.Text.Split(New String() {Environment.NewLine}, StringSplitOptions.RemoveEmptyEntries)

        Dim found As PdfFind = Nothing

        For h = 0 To parts.Count - 1
            '// if the next line is commented out, there are no memory issues
            found = currPage.Text.Find(parts(h), FindFlags.MatchWholeWord, 0)
            If (found IsNot Nothing) Then
                PdfViewer1.HighlightText(i, found.CharIndex, found.CharsCount, Color.Yellow)
                Do While (found.FindNext())
                    PdfViewer1.HighlightText(i, found.CharIndex, found.CharsCount, Color.Yellow)
                Loop
            End If

        Next

        currPage.Dispose()
        currPage = Nothing
    Next
End If