Some older versions or "unclassified" summaries are available via the NATO Standardization Office (NSO).
Every symbol in APP-11 is built from a 20-character alphanumeric code called the SIDC. The PDF provides a complete lookup table. The code breaks down as: nato app-11 pdf
Because these are standardized military documents, they are typically not freely available for public download. Access is usually managed through official channels: nato app-11 pdf