The contributions of editors to scientific papers are often more than the so-called coauthors.
I often feel this is the case for papers I have edited for others. But I also sometimes feel that I should not expect or ask to be a coauthor despite having a major role as editor. I think my feeling about this depends on whether my input is more creative or conceptual rather than merely technical. This week I have been looking at a paper which is close to my own research field, by a younger writer who would like me to be a senior coauthor. He/she may be hoping that adding my name adds some weight to the paper.
For me, the deciding point is that the paper is basically a simple, technical paper that simply needs better construction and presentation. It is useful work, but nothing that requires creative or conceptual thinking. I decided to explain to the author my modis operandi, as a volunteer, so that he/she is not too shocked when I send back the manuscript with a forest of comments and and looping arrows:
I have looked into the ms further and can send you more comments next week.
To edit any paper, including my own, I always print out a copy and write comments directly on the page, without worrying about appearance. This makes it easier to think freely about the paper as a whole (not just the part visible on a computer screen), and also means I can look at the paper while commuting to work and in other places.
My handwriting is usually difficult for others to read, but not impossible.
I will scan the pages I have looked at next week and send them to you.
I would prefer to simply have the role of volunteer reader/editor. I have a personal interest in the subject, and can occasionally do this kind of work for others. This is really your work and it is good for you to be free to accept or reject my suggestions as an independent author.
The author has to be aware that an editor may suggest something but actually does not understand what is meant in a text. In this case, the author has to think why the editor had a problem understanding the text, and find a way to make the text easier to understand.
Usually, if I know that I am not understanding something, I make a comment like 'Unclear!' or a question like 'Meaning?' in the margin.
In any case, if you find my written comments useful, I can continue looking at the paper as time permits.
I will think about your replies... and how the information fits in your paper.
Thanks, [Volunteer Editor]