The comprehensive understanding of grammar and spelling that exist in the different flavors of English such as the US and UK versions, gives you expert perspective in examination of content.
You want your document to be effective, to be taken serious (even if it is for intent of humor). Your content must be regarded, thus must be flawless.
I will proofread, and provide in a separate document, the suggested corrections. This will be for all components of grammar including spelling, use of tense, use of various figures of speech, use of various parts of speech, singular and plural context, vocabulary context.
It is more than just a spell check. It is making sure you say what you need in the most effective technically accurate way.