Hidden data embedded in a PDF file — including author, title, creation date, and software used — readable without opening the document visually.

The specification version a PDF file conforms to, from PDF 1.0 (1993) to PDF 2.0 (2017), determining which features are available.

The Universal Accessibility standard for PDF (ISO 14289) — a specification that defines requirements for PDFs to be fully usable with assistive technology.

An ISO-standardised subset of PDF designed for long-term archiving — it restricts features like encryption and external content that could prevent future rendering.

A security layer applied to a PDF that requires a password to open or to perform certain operations such as printing or copying text.

Flags embedded in an encrypted PDF that restrict operations like printing, copying, editing, or adding annotations.

Notes, highlights, shapes, or other markup objects added on top of a PDF page without modifying the underlying content.

Content placed on top of an existing PDF page — such as text boxes, images, or shapes — that is layered above the original document.

A PDF containing interactive fields that users can fill in using a PDF reader, without printing the document.

Reducing the file size of a PDF by compressing images, removing unused objects, and applying stream compression algorithms like Deflate or JPEG.

The native unit of measurement in PDF — 1 point equals 1/72 of an inch. All positions and dimensions in a PDF are expressed in points.

PDF uses a bottom-left origin — the point (0, 0) is at the bottom-left of the page, with Y increasing upward. This is the opposite of most screen coordinate systems.

A pop-up comment annotation in a PDF that collapses to a small icon until clicked — used for reviewer notes and feedback.

A PDF created by scanning a physical document — the pages are images with no underlying text layer unless OCR has been applied.

The selectable, copyable text stream inside a PDF — present in text-based PDFs and scanned PDFs that have had OCR applied.

A clickable hyperlink annotation in a PDF that links to an external URL — the standard way to add web links to PDF documents.

Extensible Metadata Platform — an Adobe-developed XML-based standard for embedding structured metadata inside PDF and other file formats.