We just released a new feature that has been on development for some time: PDF screenshots.
This is a transparent process for everybody using PagePeeker, both free and paid accounts. Our robot will understand that the URL contains a PDF document and will do a screenshot of the first page. Depending on the page aspect ratio it might look a bit distorted, but it will help understand what the PDF document is about.
Please test it out and let me know in the comments how you like it.