I assume that you are trying to download the dashboard in .pdf format, since that word goes into your title? You don't mention how much data we are talking about. Downloading large amounts of data can affect server performance. You may be downloading all the data displaying in aggregate functions on the dashboard. Plus, .pdf tends to be slower to download as the data may have to be reformatted before download.
You could try exporting the data directly from the underlying data source, depending on your needs?
Hope this puts you on the right track, otherwise reach out again.