What should you call a preface for a small document?

I have used "Summary", "Abstract", and "Overview" to provide the reader with a guide to the document and how it is organized. Generally, the notion of how to follow, understand, or "consume" the material is implicit in the wording of this section.

"Overview" is common in my industry. It is a word that is more explicit than "Introduction", putting the reader into the mindset that this is a summary of what the entire document is about, rather than just the first step into material.

If you want to be explicit on how the document should be read, there is nothing wrong with saying so in the section having this title (overview, summary, abstract).

You could call it "How to Consume this Material" or a slightly less formal rendition of this. One suggestion would be, "A Note to the Reader".

How about an introduction:

3. An initial section of a book or article, which introduces the subject material.

Besides already-mentioned preface, introduction, foreword, summary, abstract, and overview, consider précis, “A concise or abridged statement or view”.

Synopsis - noun, plural synopses [si-nop-seez] 1. a brief or condensed statement giving a general view of some subject. 2. a compendium of heads or short paragraphs giving a view of the whole. 3. a brief summary of the plot of a novel, motion picture, play, etc.