Hi Jean-Marc,
AFAIK this is only possible with the Secure View feature, which requires a Collabora Server and ownCloud Enterprise edition. I found this thread Files only to be viewed and not to be downloaded and the Press Release https://owncloud.com/news/owncloud-and-collabora-enable-setting-up-virtual-data-rooms-with-new-secure-view-feature/
But even with this feature enabled, for any users it is still possible to make screen shots, so in the end it still is possible to “copy” the pdf in some way …