PDF Text文本显示
PDF中以BT开始一个text object， 以ET结束一个text object， BT和ET不能嵌套。
Text-Showing Operators
Tj show a text string
' Move to the next line and show a text string
'' Move to the next line and show a text string
TJ Show one or more text strings, allowing individual glyph positionsiong.
