Reference
Plain-language definitions for the terms you'll encounter when working with PDF files - from compression and encryption to OCR and digital signatures.
The native interactive form system built into the PDF specification. AcroForm fields include text inputs, checkboxes, radio buttons, and drop-down menus that users can fill in without additional software.
Any markup added on top of a PDF page without modifying the underlying content. Examples include comments, highlights, sticky notes, and signature fields.
The process of reducing a PDF file's size by applying algorithms such as DEFLATE or JPEG compression to embedded images, fonts, and binary streams. Lossless compression preserves quality; lossy compression trades some quality for a smaller file.
A special type of PDF text field divided into equally spaced cells, one per character. Commonly used on government and official forms to keep entries aligned with printed boxes.
A cryptographic mechanism that verifies the identity of a document signer and detects any changes made after signing. Unlike a drawn signature image, a digital signature is tamper-evident and legally binding in most jurisdictions.
A measure of resolution for printed or rasterised images. A higher DPI produces sharper output. PDFs are resolution-independent vectors, but embedded images within a PDF are rasterised at a specific DPI.
A font whose data is stored inside the PDF file, ensuring the document renders identically on any device regardless of which fonts are installed locally.
A security layer that locks a PDF with a password. PDF encryption uses AES-128 or AES-256 algorithms. An owner password restricts editing; a user password restricts opening.
The process of merging all interactive layers - form fields, annotations, and overlays - into the static page content. A flattened PDF cannot be edited further but is safe to print and archive.
An optimisation that embeds only the characters from a font that are actually used in a document, significantly reducing file size.
A PDF optimised for streaming, where the first page is stored at the beginning of the file so it can render in a browser before the rest of the document finishes downloading.
Two approaches to compression. Lossless compression (e.g., PNG, ZIP/DEFLATE) perfectly reconstructs the original data. Lossy compression (e.g., JPEG) permanently discards some data to achieve a smaller file size.
The act of combining two or more PDF files into a single document. Pages from multiple sources are appended in sequence while preserving their original content.
Descriptive information stored in a PDF that is not part of its visible content. Metadata includes the author, title, creation date, modification date, and software used to create the file.
Technology that analyses a rasterised image of text and converts it into selectable, searchable, and copyable characters. OCR is essential for scanned PDFs that contain image-only pages.
Content placed on top of an existing PDF page without altering the original layer beneath. Text boxes, signature images, and redaction rectangles are common overlays.
A file format developed by Adobe in 1993, standardised as ISO 32000, designed to present documents consistently across all devices and operating systems. A PDF encodes text, images, fonts, vector graphics, and interactive elements in a single file.
An ISO-standardised version of PDF designed for long-term archival. PDF/A prohibits features that depend on external resources, such as encryption, live audio, and unembedded fonts.
Boundaries defined within a PDF that control how pages are rendered and trimmed. The MediaBox is the full physical page size; the CropBox defines the visible region.
Converting a vector-based PDF or its text into a grid of pixels (a bitmap image). Rasterised PDFs are no longer searchable but can be useful for consistent printing or preventing text extraction.
Permanently removing sensitive content from a PDF - text, images, or metadata - so it cannot be recovered. True redaction overwrites the data; covering text with a black box without flattening is not true redaction.
An image of a handwritten signature placed on top of a PDF page. Unlike a digital signature, a drawn signature image provides no cryptographic proof of identity.
Dividing a single PDF into multiple smaller files. Pages can be split by range, by a fixed count, or individually into one-page PDFs.
Text or an image superimposed on a PDF page, typically semi-transparent. Watermarks are used to indicate ownership, confidentiality level (e.g., "DRAFT" or "CONFIDENTIAL"), or branding.
The visual representation of an AcroForm field on a specific page. A single logical form field can have multiple widgets spread across different pages.
A reusable graphics resource in a PDF - such as an image, form, or pattern - that can be referenced multiple times within the document without duplicating its data.
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