OK, this is a known issue that we're working to resolve in Web Access 9.2. The issue is that PDFs with the applciation/octec-stream mime type cannot be viewed in certain browsers (internally, we're seeing this only on Firefox).
We'll work to resolve this issue in a future release. If you're client requires a more immediate fix, I recommend opening a case with Support (the bug is SCR128155 for your reference).
Note that this is only part of the problem however. PDFs should have a mime type of 'PDF file' or 'Adobe Acrobat Document (.pdf)' if the machine has Adobe installed. Whatever is generating this PDFs is not correctly setting their mime types. This will lead to unpredictable behavior in general with applications that use the file. I would double check with your client to see where they files are coming from. If they are being generated by a Laserfiche product, please let us know so we can resolve it.