Flattening a PDF merges all interactive layers, form fields, annotations, and transparency effects into a single static image layer. A flattened PDF is essentially a collection of page images — interactive elements no longer exist as editable objects. This is typically done before printing (to prevent printing issues caused by transparency and layers), for security (preventing editing of form data), or for archiving (creating a fixed, unalterable document).
Common reasons to flatten PDFs: a filled-out PDF form should be flattened before emailing to prevent recipients from modifying submitted data; PDF signatures are often required to be flattened in legal workflows; some printers require flattened transparency to avoid rasterization artifacts; and archival systems may require flattened PDFs for tamper-evidence.
Flattening is irreversible — once form fields are flattened, they can't be edited again. Always keep an unflattened copy. Adobe Acrobat can flatten PDFs (Print Production → Flatten Transparency). Some online tools also offer PDF flattening. FileCurve's PDF watermark tool effectively flattens watermarks into the PDF content.