should I ask that how to rotate the view for 90 degrees as the pic below,
Does the pdf-file show the same rotation, when you open it in a pdf-viewer? Do you need a special setting in the pdf-viewer to get the content upright?
Yes, pdf-file show the same rotation in a pdf-viewer. Can I get the content upright in LibreOffice? thank you!
Do you want all your pages rotated? In this case, go to Page>Page Properties and select Landscape orientation.
Note this is valid for new blank document. It will not rotate existing documents, only change the axis of the wider side of the page.
IMHO, it looks like you try to make a presentation using Draw. A better tool is Impress.
Another workaround if your goal is only to read the document. Many OS’es (you didn’t mention yours) provide the means to rotate display through driver configuration. A simpler trick (if your screen is mounted on a desk arm) is to rotate the screen; doesn’t apply to laptops.
There are free tools that rotate PDF. Once rotated, you can open it in LibreOffice.
In Draw you can change the page orientation (as @ajlittoz stated before), but content must be rotated and set in place by hand.
If you do not want to edit something, you could use a pdf-viewer directly for presentation. It should have an option to rotate the pages for the view, at least in Acrobat Reader such option exists.