In order to obtain text fragment correctly, you can either perform search using regular expressions as we also shared a code snippet in our previous response or you can extract text using rectangle which we believe you are already doing at your end as you mentioned in one of your replies.
Using both ways, you will have text fragment. Now, it is up to you whether you add highlight annotation around obtained text fragment or around segments of a text fragment. For example, in the below code, we obtained a multi-line text from your sample PDF and added highlight annotation around every segment in the obtained text fragment:
Document doc2 = new Document(dataDir + "original.pdf");
TextFragmentAbsorber tfa = new TextFragmentAbsorber(@"The\s+information\s+contained\s+herein\s+is\s+believed\s+to\s+be\s+accurate\s+as\s+of\s+the\s+date\s+of\s+publication,\s+however,\s+none\s+of\s+the\s+Blu-ray\s+Disc\s+Association,\s+its\s+Members", new TextSearchOptions(true));
foreach(var textsegment in tfa.TextFragments.Segments)
HighlightAnnotation ha = new HighlightAnnotation(doc2.Pages, textsegment.Rectangle);
ha.Color = Color.Yellow;
doc2.Save(dataDir + "PDF_Highlighting_2.pdf");
PDF_Highlighting_2.pdf (270.1 KB)
Please check in the attached PDF, you will not notice any border as well. This was generated using 21.5 version of the API. Also, as requested earlier, please do not use the code snippet given in the documentation article as it is for a particular scenario where you want to add annotation around every character. Instead, please use the code snippet which we suggested in this forum thread.
Furthermore, in case you face any issue at your end while using our suggested code, please feel free to let us know. In case we still misunderstood your requirements, we apologize in advance and request you to share an expected output PDF along with original sample code snippet that has been implemented at your end for adding a highlight annotation so that we can further proceed accordingly.