If you are writing reports to give away, then the proper length is probably four for a simple report to twelve pages for a more detailed white paper. Depending on the topic, they can be heavily illustrated with line drawings, charts or tables.
The graphic design and layout should be clean, stark and pristine with an 'academic' feel. Stay away from flashy, four-color, overly designed reports. At some primal level of consciousness, its austere 'look and feel' adds to a reports authenticity. The message is more important than the medium.
Finally, people prefer holding the physical copy of a report to read and perhaps to mark up with a pen or pencil. The finished document should be designed to be printed on 8 1/2" by 11" paper. At the same time, they appreciate that the PDF file can be saved to their computer hard drive for reprinting, if necessary.