Hi Renato,
I can see that you are using PHP 7.2 - can you tell me a bit more about the installation? Is it using Ubuntu 18.04 as well? Does your site have any customizations, such as a custom theme, or other custom development? What is the full version of AtoM listed in Admin > Settings? Are these PDFs uploaded locally, or are they linked via URL?
When you were performing the upgrade steps, did you
run the upgrade task, and restart all services after? I think that later versions of Atom no longer use digitalobject/view as a path - in version 2.3 we combined the digital object browse page, the search, advanced search, and the description browse pages into a single page type. Can you provide the exact steps you are following to reproduce this error? For example, are you seeing this while in the digital object browse and clicking a result, or while on the view page of a description and trying to click on the reference image of a digital object? Are you logged in or out when trying this?
Hopefully with more information we can help you troubleshoot this issue!
Cheers,