Thumbnail can be generated from a bitmap (.BMP) or a JPEG (*.JPG) or even a EMF (Encapsulated Meta File).
When thumbnails are created and embedded into your PDF files, you will see the thumbnails displayed on the left column of your PDF Reader or Viewer (usually Adobe Reader).
With this handy software utility, you can decide whether to show thumbnails when PDF file is opened.
SYSTEM REQUIREMENT
Supported platform: Microsoft Windows 98/98SE/ME/NT4/2000/XP/Vista/Server 2003/Server 2008/Server 2012 and Windows 8/7 32-bit and 64-bit.