Thuật ngữ PDF

A4, Letter, Legal (page sizes)

The three most common PDF page dimensions — A4 (European standard), Letter (US default), and Legal (longer US document size).

AES Encryption

The Advanced Encryption Standard — the cipher used to protect modern password-protected PDFs. AES-256 is the strongest commonly deployed version.

Annotation (PDF)

A note, highlight, shape, or comment added on top of a PDF without modifying the underlying content — used for review, markup, and collaboration.

Bates Numbering

Sequential identification numbers stamped onto each page of a PDF, commonly used in legal discovery to give every page a unique reference.

Bookmarks (PDF)

A navigable outline inside a PDF — the table of contents that appears in the sidebar of most PDF readers, letting you jump to specific sections.

Compression (PDF)

Techniques for reducing PDF file size — by re-encoding images at lower quality, removing unused objects, or both.

CropBox & MediaBox

Two of the PDF boxes that define what part of each page is visible and printable — the MediaBox is the physical page, the CropBox is the visible window.

Digital Signature (PDF)

A cryptographic signature embedded in a PDF that proves who signed it and that the file has not been modified since.

DPI (Dots Per Inch)

A measure of image resolution — the number of dots of color packed into one inch of space. Higher DPI means more detail, larger file size.

Flattening (PDF)

Permanently merging interactive elements — form fields, annotations, signatures — into the static page content of a PDF so they can no longer be edited.

Font Embedding

Packaging the fonts used in a PDF inside the file itself, so the document renders correctly on any device even if those fonts aren't installed.

Form Fields (PDF)

Interactive input areas in a PDF — text boxes, checkboxes, dropdowns, and signature fields — that users can fill in electronically.

Ghostscript

An open-source PostScript and PDF interpreter used widely for PDF conversion, compression, and printing — the engine behind many PDF tools.

Hyperlink (PDF)

Clickable links embedded inside a PDF that jump to web addresses, other pages in the same document, or external files.

Layer (PDF)

An optional content group in a PDF — text, images, or annotations organized into independent layers that can be shown, hidden, or printed separately.

LibreOffice

A free, open-source office suite (Writer, Calc, Impress) that can convert between Word, Excel, PowerPoint and PDF — widely used as a server-side document conversion engine.

Merge PDF

Combining two or more separate PDF files into a single document, preserving their original content and order.

Metadata (PDF)

The hidden information embedded in a PDF about the file itself — title, author, subject, keywords, creation date, and the software that produced it.

N-up printing

Laying out multiple PDF pages on a single physical sheet (2-up, 4-up, 9-up) — for compact printing, handouts, or saving paper.

OCR (Optical Character Recognition)

The technology that reads text from images or scanned PDFs and turns it back into searchable, selectable characters.

PDF

The Portable Document Format — a file format that preserves layout, fonts, and graphics so a document looks identical on every device.

PDF Encryption

Protecting a PDF with a password, typically using AES-128 or AES-256 encryption, so the file cannot be opened or modified without the key.

PDF File Size

The byte count of a PDF file — driven mostly by embedded images and fonts. Reducing file size usually means compressing images, not text.

pdf-lib

A pure-JavaScript library for creating and modifying PDF documents entirely in the browser — no server round-trip needed.

PDF.js

Mozilla's open-source JavaScript PDF renderer — the engine behind Firefox's built-in PDF viewer and most browser-based PDF preview tools.

PDF/A

An ISO-standardized variant of PDF designed for long-term archival — fonts are embedded, external dependencies are disallowed, and the file will render identically decades from now.

qpdf

A command-line tool for inspecting, modifying, encrypting, and repairing PDF files — lighter-weight than Ghostscript and ideal for structure-only operations.

Rasterization

Converting vector content (text, shapes, lines) into a grid of pixels — a raster image. Often a deliberate step for redaction or compression; sometimes an unwanted side effect.

Redaction

Permanently removing sensitive text or images from a PDF so they cannot be recovered by copying, searching, or inspecting the file.

Split PDF

Dividing a single PDF into two or more smaller files, either by page range or one page per file.

Tagged PDF

A PDF that includes hidden structural tags — headings, paragraphs, lists, tables — that describe the content's logical hierarchy for screen readers and data extraction.

Tesseract

The most widely used open-source OCR (optical character recognition) engine — originally developed at HP, now maintained by Google.

Vector Graphics

Images defined by mathematical paths and shapes — they scale to any size without losing quality, unlike pixel-based raster images.

Watermark

A semi-transparent text or image overlaid on every page of a PDF — used to mark drafts, indicate ownership, or discourage unauthorized copying.