PiPO M9 pro OTA - Manual (Page 25)

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You can page up or page down by sliding your finger right or left.
Tap or the menu button to check the sub-menus. You can set the font
size, Go to Page jump; check your bookmarks, set the auto flip time, etc.
6.6 Sign function
Tap the ―sign‖ icon to open this.
You can make notes and sign your signature on PDF, Picture file.