This is just a quick example to show the differences between a Tableau sheet and a Tableau Dashboard, specific to the goal of printing or using PDF output. By definition, Tableau dashboards are best considered "single page entities". In contrast, a Tableau sheet (or view) can span multiple pages. This makes logical sense.
In Tableau desktop, for an individual sheet, you can specify portrait or landscape mode for that sheet, from the File>Page Setup command. This setting will be honored once you publish to Tableau Server.
For example, the following link will auto-generate a PDF of a multi-page report. Notice that the output is already set to "landscape" mode:
Click here to view the multi-page PDF report
Whereas, this second link is the same exact information, but placed onto a dashboard which has been sized to 1100 x 850 (letter landscape). It will only print one page, and thus, a good chunk of the information is cropped and not visible:
Click here to view the single-page PDF dashboard
Again, this is all quite logical: it doesn't really make any sense to print a multi-page dashboard. Else, why call it a dashboard in the first place?