From the Knowledgebase

Invalid PDF Structure

If you see 'Invalid PDF Structure' where you expected the PDF to display, this may be due to server configuration problems. Either the encrypted file isn't being streamed correctly to the browser (maybe due to gzip being applied incorrectly), or the decryption key in the browser is stale.

Reload the page to check it's not a one-off. Clear any cache (e.g. through a caching plugin), and ideally disable the cache for the page containing the secure PDF going forward. That should ensure the latest decryption key is used and may fix the problem.

In any case, please just get in touch for our help in diagnosing the problem: