As the title says,
I'm essentially looking for a way to generate a thumbnail from a .DOC, .DOCX, or .PDF file uploaded to the linux machine.
I understand ImageMagick can handle .PDF to .PNG conversion and is made even easier with PHP's IMagick but I am unsure about .DOC or .DOCX. I think I was told a headless version of OpenOffice can do this.
Haven't tested it myself yet however ooo-thumbnailer can apparently make thumbnails of DOC, DOCX and PDF files from version .31 and up.
The project is available from here: https://launchpad.net/ooo-thumbnailer
This is a subject that I've spent a long time searching myself and it seems that this is probably your best bet.