How to get the character '±' correctly？

I use the function 'AVDocGetPageText' to get the '±' character in the page, but keep returning '? '

My code is as follows：

PDTextSelect textSelect = PDDocCreateTextSelect(pdDoc, 0, &rect);
bool ret = AVDocSetSelection(avDoc, ASAtomFromString("Text"), textSelect,true);
ASAtom format = ASAtomFromString("Text");
string title = "";
AVDocGetPageText(m_avDoc, vecPages[i], textSelect, format, TextSelectProc, &title);

void TextSelectProc(ASAtom format, void *buf, AVTBufferSize bufLen, void *clientData)
{

          // Look at the memory of *buf and it returns 3f

}