i tested following codes, and it works:
m_annot_pos = m_layout.vGetPos((int) event.getX(), (int) event.getY());
m_annot_page = m_layout.vGetPage(m_annot_pos.pageno);
Page page = m_doc.GetPage(m_annot_page.GetPageNo());
page.ObjsStart();
int ichar = page.ObjsGetCharIndex(new float[]{m_annot_pos.x, m_annot_pos.y});
int start = page.ObjsAlignWord(ichar, -1);
int end = page.ObjsAlignWord(ichar, 1);
String result = page.ObjsGetString(start, end);
Log.e("TAP", result + " " + ichar + " " + start + " " + end);
if not work yet. it maybe following issue:
1.you have not active any license, even demo license.
2.the PDF page is pure image page, the text is pixel image, not fonts.