I have seen this on two other Confluence instances we manage, specifically with PDF files. We spent quite a lot of time working with Atlassian, but Atlassian has not reproduced, so our service desk ticket, AFAIK has not been officially turned into a ticket for me to link to.
We are using Confluence 5.9.x in both cases, on Linux with MySQL.
You might try to go through the support process, and perhaps they can reproduce your problem.
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