BuildingWorks 18.4.2 - released 23rd October 2018 now has built in tools to allow you to rotate PDF files. See Update News for more info.
It is quite common that PDF files are created in portrait. When you import these into BuildingWorks they come in sideways. You cannot rotate the imported image in BuildingWorks because its orientation is coded into the imported image.
You need to rotate and save the pdf prior to importing into BuildingWorks. You can do this with a full pdf editing program such as Adobe Acrobat. If you just have a pdf viewer (Adobe Acrobat Reader for example) , this will NOT save an image you have rotated. To get around this you can use the pdf rotation tool on this website: