Last tested: Mar 18, 2020
This error means that after the queries finished running on the page and was ready to render, the process of saving the finished page as a PDF either failed or was still processing after 5 minutes of trying to create the PDF. This will often happen if the dashboard is really large or has a lot of data, and Chromium struggled to save the full page.
If you notice slow performance in your browser when viewing the dashboard, it may be that you need to make the dashboard smaller. If you host your looker instance, you may need to allocate more resources to the server so that Chromium has enough memory and CPU time to run without hanging.
There is also an experimental feature called Render Long Tables, which can reduces the dashboard PDF rendering times for dashboards containing tables with many rows.