SQLite explicitly encourages using it as an on-disk binary format. The format is well-documented and well-supported, backwards compatible (there's been no major version changes since 2004), and the developers have promised to support it at least until the year 2050. It has quick seek times if your data is properly indexed, the SQLite library is distributed as a single C file that you can embed directly into your app, and it's probably the most tested library in the world, with something like 500x more test code than library code.

Unless you're a developer that really understands the intricacies of designing a binary data storage format, it's usually far better to just use SQLite.