I've been doing both proofreading and editing as long as I could read. I have been able to proofread simple items like Engineering Specifications (nothing mathematical, making sure that Units of Measure match up), to more complex items like Property Evaluation Reports and Operating and Maintenance Manuals. While many do segregate the two, when I proofread something, I also do actually read the document. If there are questions I will ask them, which results in a easier to read, clearer, document.