You can prevent this situation from happening in the first place by rotating and saving the images, but if that it too much then you can always use your PDF viewer/reader to do this provided your software supports this feature. When you create a PDF from your images, or from online resources such as websites, sometimes the PDF may display the content in the wrong direction.
PDF (.pdf) is an acronym for Portable Document Format. Best PDF Readers and Viewers Software for Windows for Viewing/Reading, Basic Editing, and Annotating PDF Documents