It strikes me that most of us were trained to prepare documents to be read offline - that is, in hard-copy format - and that this generally means that documents are prepared in "portrait" format.
However, increasingly, we are all working on PCs, which typically have screen that are in "landscape" format. As a result, I find most documents quite difficult to read online.
The solution, I think, is to write more documents in landscape format.
I find it quite useful, when reading landscape documents online, to put the programme into full page and full screen mode. Popular programmes such as MS-Word and Adobe reader both provide both of these options. This makes it easier to page up and down through the document, rather than having to scroll through it. I find paging much more predictable and natural than scrolling, especially where there are columns or other helpful layout features employed. When paging, the author can control the experience of the layout much more like they can in conventional print medium.
Of course, it helps a lot if the document has been laid out with that in mind. One of the disadvantages of landscape documents is that the line lengths tend to get too long for comfort. An easy solution to this is to use a column layout. Someone once told me that the optimum number of characters per line to use for online reading is 33. From a little experimenting on my part, I have found that this optimum translates roughly into three columns on an A4 landscape page with a font-size of 12. Three columns offers the added advantage of flexibility for graphics and tables and the like - they can typically span 1, 2 or all three columns.
So lets all start writing documents that are easier to read online, and print less of them out. And please feel free to add your own views and tips below.