Nevermind.
TLDR: client antivirus was blocking pdf file http path, check antivirus config.
Opening chrome dev tools i've found out it was an HTTP 403 error to the pdf file.
For each pic or pdf dynamically loaded, there is a direct HTTP GET from the client itself.
The issue was caused by the client's antivirus software (web protection module) blocing for some reason all requests to pdf files.
Changing AV configuration on client side solved the issue, so definetly was not an Ignition Perspective problem.