PDFs created with other applications (f.ex. Autocad), positioning in Adobe Acrobat (9.0, PC)

Hello!

Why often PDFs, which are created with other applications (f.ex. Autocad), are often posited 90 degrees compared to the original, when they`ve been brought to the Adobe Acrobat (9.0 Pro, PC)? That happens every time when you open the document, even though you have saved it "correctly rotated"...