Determining if a File is a PDF in Rust

In my work generating BI (Business Intelligence) reports as PDFs with Apache FOP (the Java XSL-FO to PDF engine), I often need to verify that an output file is actually a PDF before further processing. A reliable way to do this is to inspect the file’s “magic bytes” – the fixed signature at the start of the file. Magic bytes (also called file signatures or magic numbers) are small sequences of bytes placed at the beginning of a file to identify its format. Because these signatures are defined by the file format itself and deliberately chosen to be unique, checking them is a fast and reliable way to recognize a PDF (or any file type) regardless of its extension.