Most of what you want is in the documentation - but there is a lot of documentation!! so I will give you a guide
First of all please look athttps://www.jdownloads.net/documentation-for-v3-9/modules-plugins-v3-9/using-the-content-plugin-in-articles-and-modules
First I suggest you use one of the 'short' layouts. You can set up which layout is used in Options - Content Plugin tab in the Default Layout option
"Can I skip "Download Details" ? I would like the person to download directly instead of getting download details page which use the layout : "Example Details Layout", "Standard Details Layout - Full Info" etc.."
To use a direct Download Go to Options - Front End Tab and see the Download sequence section. for more info seehttps://www.jdownloads.net/documentation-for-v3-9/options-configuration-v3-9/a-tutorial-on-the-frontend-optionshttps://www.jdownloads.net/documentation-for-v3-9/faq-v3-9/download-actions-direct-via-summary-page-or-via-download-details
I would like only registered users to be able to download. Should I set this in "Downloads" > "My Download item" > "permissions" ?
The simplest way of resoving this is to go to jD Options and select the Permissions tab. Select the Public User Group and change the permission setting of Download to Inherited - NEVER
set to Denied. Then go to the Registered UG to Allowed. (little green ticks will be shown against the UGs you have changed) Now do a Save
( the green ticks dissappear) These permissions will then propogate through all yor jD categories and Downloads.Users who are not logged in will still be able to see the Downloads but will not be able to Download.
For more info see https://www.jdownloads.net/documentation-for-v3-9/access-user-group-permissions-v3-9/an-initial-view-of-permissionshttps://www.jdownloads.net/documentation-for-v3-9/access-user-group-permissions-v3-9/user-groups-settings-overview-2
Is it possible to restrict the number of downloads per user ? Let's say out of 10 downloadable image, he can download only 3 per day.
Here you will need to access User Group Settings in the Backend. As your users are in the Registered UG the in User Groups Settings select the Registered UG.Now select the Limits Tab. You will see limits for Daily, Weekly and Monthly. Obviously you set the value 3 in the Daily Download Limit option. I suggest you do not use any of the other options below the Monthly limit option.
Also there is a note "When you setup user group limits, it is very important that you activate jDownloads logging in Options - Global Settings, otherwise this function will not work properly. Also remember that the duration for keeping log data sets should be set in the jDownloads system plugin to avoid excessive database growth." The Default setting for the system plugin is 30 days so that is OK, and the default setting for 'Activate Log Function' is Yes. But please check.