It's not PSPad related question. Problem is in your PDF. PDF is final exported document for print, not for back work. Some characters can be "painted" e.t.c.
The solution is to find another PDF, use OCR tool like Abbyy Fine reader -- <https://forum.pspad.com/read.php?2,73848,73849> PSPad freeware editor https://www.pspad.com