То be able to add a file or a link to an activity, you need to have a Project Manager role or higher, and to be owner of the project the activity blongs to.
Adding files to an activity
To add a file to an activity:
Open a project and select an activity.
Select the … menu in the activity’s upper right corner.
Select View attachments.
4. In the Files dialog box, select Add files > Drag & Drop / Browse for files.
5. Open a file and select Upload.
Adding links
To add a link to an activity:
Open a project and select an activity.
Select the … menu in the activity’s upper right corner.
Select View attachments.
In the Files dialog box, select Add links.
Paste a link in the Link to External File box and select Add.
Note: The files and links you add on activity-level are immediately added on project-level as well, and can be found on the Files tab when you open a project.