Unless your PDF documents are in the right orientation, else you will need to turn your computer upside down or move yourself 90 degrees to the left and right side of your computer screen to view your PDF files.
Sometimes, you got PDF files which comes in landscape format that all the pages are not properly oriented. So, you will have to turn them arounds in the correct angle using the PDF rotation feature in order to view it in the right orientation every time you open your PDF files.
Here is a useful and handy tool that you can use to rotate your unoriented properly pages and save the files into a proper oriented PDF pages.
RotatePDF is a free online tool that you can use to rotate PDF format file and save online to download for free. What you need to do is just select the PDF document that you want to rotate on your computer, select the right rotation angle you want your pages to be rotaed and click on the Rotate PDF button.