PDF Glossary

Clear definitions of essential PDF terms: PDF/A, OCR, linearization, Bates numbering, watermark, metadata and more.

Flatten: Flattening merges a PDF's interactive layers — form fields, annotations, and optional content layers — directly into the page content. The document keeps its exact visual appearance, but its fields can no longer be edited or filled in. It is commonly used to lock a completed form before archiving or distribution.
Encryption: PDF encryption protects a document's content using a cipher (usually AES 128- or 256-bit) tied to a password. A distinction is made between the user (open) password, required to view the file, and the owner (permissions) password, which restricts printing, copying, or editing. Without the matching key, the content stays unreadable.
Compression: Compression reduces a PDF file's size by optimizing its components: downsampling and re-encoding images, removing redundant data, and compressing content streams. It can be lossless, preserving full quality, or lossy, slightly degrading images for a greater size reduction. The goal is to make files easier to share and store.
CropBox: The CropBox is one of a PDF page's bounding boxes; it defines the visible region that is actually displayed and printed. Unlike the MediaBox, which describes the full physical dimensions of the medium, the CropBox can hide unwanted margins or edges without deleting the underlying content. Adjusting the CropBox therefore crops a page non-destructively.
Watermark: A watermark is a semi-transparent text or image overlaid on the pages of a PDF, often diagonally. It is used to indicate a status ("Confidential", "Draft"), to mark a document's ownership, or to discourage unauthorized copying. The watermark can be placed above or below the existing content.
Imposition: Imposition is the arrangement of several pages on a single sheet so that they appear in the correct order once folded and bound. In booklet mode, pages are reordered and paired two by two to form a signature folded down the middle. N-up mode instead groups several scaled-down pages onto one sheet, for example for economical printing.
Linearization: Linearization, also known as "fast web view", reorganizes a PDF's internal structure so that its first page can be shown before the whole file has downloaded. Objects are reordered and a special index is added to allow progressive rendering and byte-range access to a specific page on a remote server. This is especially useful for large documents viewed online.
Metadata: A PDF's metadata is the descriptive information attached to the document without being part of its visible content: title, author, subject, keywords, creating application, and creation or modification dates. It is stored in the document information dictionary and often in an XMP stream. Viewing or clearing these fields helps to index a file properly or to strip sensitive data from it.
Bates numbering: Bates numbering is an indexing system that stamps a unique, sequential identifier on every page of a set of documents, often with a prefix and a fixed number of digits. Originating in US legal and administrative practice, it makes it easier to reference, catalog, and track exhibits during a proceeding. Each page thereby becomes unambiguously citable.
OCR: OCR (Optical Character Recognition) is a technology that analyzes the image of a scanned document to extract its text as editable characters. Applied to an image-based PDF, it adds an invisible text layer aligned with the words, making the document searchable and selectable. It is essential for turning scans and photographs of documents into searchable files.
PDF: PDF (Portable Document Format) is a file format created by Adobe that has become an open ISO standard (ISO 32000). It describes a document's layout, text, fonts, images, and vector graphics in a fixed way so that it displays and prints identically regardless of the system used. It is today the reference format for exchanging finalized documents.
PDF/A: PDF/A is a standardized version of PDF (ISO 19005) designed for long-term electronic archiving. It requires that everything needed to render the document faithfully be contained within the file itself — embedded fonts and color information — and forbids externally dependent elements such as links to files or encryption. This ensures a document will remain identically legible decades from now.
Embedded font: An embedded font is a typeface whose glyph data is included directly inside the PDF file, ensuring correct display even on a system that does not have that font installed. To keep the file small, only a subset containing the characters actually used is often embedded. Font embedding is essential to the portability and faithful archiving of documents.
Electronic signature: An electronic signature attaches proof of identity and integrity to a PDF document, making it possible to verify who signed it and that it has not been altered since. The most robust digital signature relies on public-key cryptography and a certificate, whereas a so-called simple signature may amount only to placing an image of a handwritten mark. Its legal weight depends on the level of assurance and the applicable regulatory framework.