title_body: I would like to view full cover of converted pdf ebook originally purchased on Kindle Store
upvoted_answer: You can create an image of the page you desire to be displayed as the cover, and then have calibre use that image as the cover.
  
  If you already have an image, then open the metadata editor and on the right side of that pop-up notice that under the cover image is a button to browse. Use that to select your desired image.
  
  In your case, you don't yet have an image and I understand that you want to use a particular page of the e-book. What follows is a simple one-liner for the linux (debian) command-line using mutool from package mupdf-tools; if you don't use linux, it may be that the exact same package / program exists for your operating system, or you can look for a corresponding command...
  
  mutool draw -o "OUT-%d.png" IN.pdf PAGE-NUMBER
  
  
  ... where OUT is your desired output filename, IN.pdf is your ebook, and PAGE-NUMBER is the page number of your desired cover.