Missing \begin{document}

The preamble of your document is the stuff before \begin{document}; you put \usepackage commands and your own macro definitions in there. LaTeX doesn’t like typesetting anything in the preamble, so if you have:

the error is inevitable and the solution is simple — judicious use of comment markers (‘%’) at the beginning of a line, moving things around, providing something that was missing … or switching to the correct file.

The error may also occur while reading the aux file from an earlier processing run on the document; if so, delete the aux file and start again from scratch. If the error recurs, it could well be due to a buggy class or package.

However, it may be that none of the above solves the problem.

If so, remember that things that appear before \documentclass are also problematical: they are inevitably before \begin{document}!

Unfortunately, modern editors are capable of putting things there, and preventing you from seeing them. This can happen when your document is being ‘written’ in Unicode. The Unicode standard defines “Byte Order Marks” (BOM), that reassure a program (that reads the document) of the way the Unicode codes are laid out. Sadly ordinary LaTeX or PDFLaTeX choke on BOMs, and consider them typesetting requests. The error message you see will look like:

! LaTeX Error: Missing \begin{document}.
(Those s are your operating system’s representation of an unknown character; on the author’s system it’s a reverse video ‘?’ sign.)

You can spot the BOM by examining the bytes; for example, the Unix hexdump application can help:

$ hexdump -C 
00000000  ef bb bf 5c 64 6f 63 75 ...
The 5c 64 6f 63 75 are the “\docu” at the start of (the ‘real’ part of) your document; the three bytes before it form the BOM.

How to stop your editor from doing this to you depends, of course, on the editor you use; if you are using GNU Emacs, you have to change the encoding from utf-8-with-signature to ‘plain’ utf-8; instructions for that are found on the “stack overflow” site

(So far, all instances of this problem that the author has seen have afflicted GNU Emacs users.)

Fortunately XeTeX and LuaTeX know about BOMs and what to do with them, so LaTeX using them is “safe”.