The article is written about Deskzilla, but it applies to JIRA Client as well.

Problem

It is impossible to access and read attached documents when working offline.

Solution

Deskzilla can open attachments when you are offline, but only if they have been previously downloaded. At the moment, Deskzilla does not download attachments automatically because there can be a lot of documents attached to all issues, and downloading a lot of information at a time can stress the server and consume a lot of bandwidth.
 
When you are online and have Deskzilla synchronize with the server:

  1. Run a query to show all issues that may have attachments you may need when you are offline.
  2. Press Ctrl+A to select all issues in Issues table.
  3. If any of the issues are not fully downloaded (partially downloaded issues have half-filled box icon in the Flags leftmost column), right-click and select Download on the context menu (you can use this action only for the partially downloaded issues).
    Deskzilla downloads issues details, including whether there are any attachments to issues.
  4. After all issues have been fully downloaded, right-click and select Download Attachments on the context menu to download all attachments for all selected issues. If this action is disabled, there are no attachments that were not downloaded.
     
    In the upcoming Deskzilla version, partially downloaded issues will already contain information about attachments.