Flattening in cybersecurity, particularly regarding PDF files, refers to the process of merging all layers, annotations, and interactive elements into a single, static layer. This makes the document non-editable and prevents changes to its content or structure. Flattening is often used to secure documents before sharing, ensuring that recipients cannot modify form fields, comments, or other interactive features. This process is crucial in legal, financial, and official documentation to maintain integrity and authenticity. However, once a PDF is flattened, original interactive elements are permanently lost and cannot be restored.