Jamie Edy asked this question some time ago, and John Smith reminded
me about it recently.
Here is my opinion on the question of references to file formats in
the user guides.
* When referring to the format in text, capitalize the 3-letter term
and do not include a period (full stop) or an asterisk (*); for
example: PDF, ODT, ODS, DOC. This is the most common usage in our
books.
* When referring specifically to the file *extension*, use the form
.pdf, .odt, .ods, .doc, in lower case and without an asterisk. One of
the few places where this form is used is in the lists of files
formats that LibreOffice can open and save to, mainly in appendix B of
the Getting Started guide.
When in doubt, ask on the list.
--Jean