The PDF export for dashboards built into Splunk is not really pretty, and there's sadly no way to improve on it.
You can do a workaround, like setting up a system that opens a dashboard in actual browser and then uses some browser plugin to screenshot it. There's actually an app for that in development by Splunk PS, but it's not public yet. You might be able to get by contacting support...
Hope that helps - if it does I'd be happy if you would upvote/accept this answer, so others could profit from it. 🙂
thanks for your answer.
is aware of whether it is possible to send several queries by building a report similar to a dashboard from splunk report.
🙂