The Open Logic Text may be one of only two open source logic texts, but there are other open, and even more non-open but still freely available textbooks of various levels. We’re keeping track of them on the Open Logic wiki:
The most popular intro level text is P. D. Magnus‘ forall x, which deserves special mention since it is, like the OLT, open source. The LaTeX code is available, which means there’s not just a PDF you can give your students, but you can make a PDF to your own specifications, with your own additions or changes. Other free texts are available only as PDFs or online in HTML format, and most of them are not provided under a license which allows you to distribute them, let alone make changes to them.
If you are the author of teaching resources — from handouts to complete textbooks — and you want others to be able to make use of them, please (a) provide the source code and (b) make them available under an open license, such as CC-BY.