How can show pdf preview first image?
You have to firstly install imagemagick in your operating system.
for Ubuntu please follow below procedure:-

(1)https://medium.com/@sanjaywrites/install-latest-version-of-imagemagick-in-ubuntu-16-04-c406ddea1973

(2) https://www.enovate.co.uk/blog/2015/12/16/how-to-install-imagemagick-from-source-on-ubuntu-14.04

For window download imagemagic exe file.
https://imagemagick.org/script/download.php

once your Operating System have magicmagick  then install these module pdfpreview https://www.drupal.org/project/pdfpreview

and do a needful change as shown in image

and on your view  do a needful change as shown in image