When there is no ToUnicode CMap, the text from the Simple Font instance (TrueType or Type1 font) should be extracted by mapping the glyph name to its corresponding charcode according to Adobe Glyph List. Additionally, the Differences array should be included in these calculations when there is custom encoding. The current implementation of RadPdfViewer makes ToString to the original char id byte value which leads to wrong characters. 

More information about this may be seen in the second point of section "5.9.1Mapping Character Codes to Unicode Values" on page 470 in PdfReference 1.7.