Salesforce does not guarantee all file types and their contained features will properly generate previews in Content.
Please refer this doc detailing the limitations of Content file previews.
Limitation around PDF files
Large numbers of pages. Exact number is dependent on their content but if the count is high (100+) it may cause the preview generation ...
I created a rollup with DLRS using the ContentDocumentLink object as David Reed recommended above, and it worked perfectly. I then was able to create a normal validation rule requiring the rollup was greater than 0.