@Matteo_Marcato These are the ways to obtain MS Office Online server (which is successor of the web server) as per Microsoft:
Office Online Server can be downloaded from the Volume Licensing Service Center (VLSC). Office Online Server is a component of Office; therefore, it will be shown under each of the Office product pages including Office Standard 2016, Office Professional Plus 2016, and Office 2016 for Mac Standard.
For customers whose licenses qualfiy for OOS, but cannot obtain it through the VLSC, the following actions are possible:
- VL Open customers can contact their Support Center.
- Customers who purchased O365 online from Microsoft can submit a request from their Office 365 admin center or contact support.
No free downloads unless you already paid for another license.
Source: https://docs.microsoft.com/en-us/officeonlineserver/office-online-server