You're right, there is no problem mixing file formats in a library. it's neater if it's standardised, but a library of, say, 'drawings' could collate all photographs of type 'drawing' and all uploaded documents of type 'drawing'.
The PDF upload process could be modified to create a JPEG thumbnail so that these could be shown with the image thumbnails.
But that's the issue, all document view pages would need complete re-coding to provide this.
Is there enough demand to justify the work this would require? There are some incredibly generous contributors of some really high quality material on here. Would improving (and I quite accept this would be an improvement) the way drawings and plans are catalogued encourage more use of the site? After all the main reason this site exists is as a permanent resouce and archive of relevant information so I am quite happy to entertain more improvements if there is a concensus that it would encourage more material to be submitted.
my orders are to sit here and watch the world go by