PDF glossary
What the words mean, and why each one matters when you edit or share a PDF.
- .notdef glyph (tofu)
- The fallback shape a font draws when it has no glyph for a code, usually an empty box nicknamed tofu.
- AcroForm (fillable form)
- The PDF standard for interactive form fields like text boxes, checkboxes and dropdowns.
- ActualText
- A marked content property that tells readers what text a sequence of glyphs really represents.
- Annotation
- An object placed on top of a PDF page, like a comment, highlight, link or form field, separate from the page content.
- Baseline
- The invisible line letters sit on; descenders like g and p hang below it.
- CID font (Type 0)
- A font format that uses multi byte codes, needed for large character sets like Chinese, Japanese and Korean, and common in modern exports.
- Clipping path
- A shape that limits where drawing appears; anything outside it is invisible.
- Content stream
- The list of drawing instructions that paints a PDF page: text, lines, shapes and images, in order.
- Continuation page
- A new page inserted after the current one to hold content pushed off the bottom by an edit.
- CropBox
- The part of a PDF page that readers display and print; content outside it is hidden but not deleted.
- Cross reference table (xref)
- The index at the end of a PDF that tells readers where each object starts in the file.
- Current transformation matrix (CTM)
- The matrix that maps drawing coordinates onto the page, handling scaling, rotation and translation.
- Digital signature
- A cryptographic signature that proves who signed a PDF and that it hasn't changed since.
- DPI (dots per inch)
- Image resolution: how many pixels are used per inch of the printed page.
- Electronic signature
- Any electronic sign of agreement, such as a drawn, typed or pasted image of your signature on a document.
- Em square
- The design grid of a font; its height equals the font size.
- Embedded font
- A font whose glyph outlines are stored inside the PDF, so the document looks the same on every device.
- Flate compression
- Lossless ZIP style compression used for most PDF streams, including text content.
- Flattening
- Turning annotations, form fields or layers into ordinary page content so they can't be changed or hidden.
- Font metrics
- The measurements of a font: glyph widths, ascender, descender, cap height and more.
- Font subsetting
- Embedding only the glyphs a document actually uses instead of the whole font, to keep the file small.
- Form XObject
- A reusable group of drawing instructions that can be placed on a page, often for headers, logos or repeated elements.
- Glyph
- A single drawn shape in a font, such as the letter a, the fi ligature or a bullet.
- Graphics state
- The current settings used while drawing: transformation, colours, line width, fonts, clipping and more.
- Image XObject
- An image stored once in a PDF and drawn on a page with the Do operator.
- Incremental update
- Appending changes to the end of a PDF file instead of rewriting it, so the original bytes remain intact.
- JBIG2 and CCITT
- Compression formats for black and white images, used by document scanners to make tiny files.
- JPEG (DCT) compression
- Lossy image compression used for photos in PDFs, stored with the DCTDecode filter.
- Kerning
- Adjusting the space between specific pairs of letters, like A and V, so text looks evenly spaced.
- Leading (line spacing)
- The vertical distance between the baselines of consecutive lines of text.
- Ligature
- A single glyph that joins two or more letters, such as fi or fl.
- Linearization (fast web view)
- Arranging a PDF so the first page can be displayed before the whole file has downloaded.
- MediaBox
- The full physical size of a PDF page, such as A4 or US Letter.
- Object streams
- Compressed containers that pack many small PDF objects together to reduce file size.
- OCR (optical character recognition)
- Software that recognises letters in an image and turns them into text.
- Page rotation
- A page setting that tells readers to display the page turned by 90, 180 or 270 degrees.
- Paragraph detection
- Reconstructing lines, paragraphs, lists and columns from the positioned glyphs in a PDF.
- PDF compression
- Making a PDF file smaller, losslessly by repacking data or lossily by reducing image quality.
- PDF encryption
- Protecting a PDF's contents with a password, using algorithms such as AES 256.
- PDF metadata
- Information about a PDF such as title, author, creation date and the software that made it.
- PDF permissions
- Flags in an encrypted PDF that tell readers whether printing, copying and editing are allowed.
- PDF/A
- An ISO standard version of PDF for long term archiving, with every font embedded and no external dependencies.
- Points (PDF units)
- The unit of measure in PDF: 72 points equal one inch, about 2.83 points per millimetre.
- Rasterization
- Turning vector content like text and shapes into a grid of pixels.
- Redaction
- Permanently removing sensitive content from a document, usually shown as a black box where it was.
- Reflow
- Rewrapping text and moving the content below it when an edit makes a paragraph longer or shorter.
- Scanned PDF
- A PDF whose pages are images of paper documents, with no real text unless OCR was applied.
- Soft mask (SMask)
- A greyscale image that gives another image transparency, like the alpha channel of a PNG.
- Stamp
- A mark added to a page to show a status or approval, like PAID, APPROVED or a company seal.
- Standard 14 fonts
- Fourteen fonts every PDF reader must provide: Times, Helvetica and Courier in four styles, plus Symbol and ZapfDingbats.
- Synthetic bold
- Making regular text look bold by stroking the outline of each glyph, used when no real bold font is available.
- Tagged PDF
- A PDF with a structure tree that labels headings, paragraphs, lists and tables for screen readers.
- Text operators (Tj and TJ)
- The PDF operators that draw text: Tj shows a string of glyphs, TJ shows glyphs with individual spacing adjustments.
- Text render mode
- A setting that says how glyphs are painted: filled, stroked, both, invisible, or used as a clipping path.
- ToUnicode map
- A table inside a PDF font that maps glyph codes back to Unicode characters, so text can be searched and copied.
- Transparency and opacity
- Settings that let content show through what's drawn on top, from fully clear to fully solid.
- Type 3 font
- A font whose glyphs are defined by ordinary PDF drawing instructions rather than a standard font program.
- Vector graphics
- Shapes described by lines and curves rather than pixels, so they stay sharp at any zoom.
- Watermark
- Text or an image placed across pages to mark status or ownership, like DRAFT or CONFIDENTIAL.
- White box editing
- Hiding old text under a white rectangle and typing new text on top, instead of editing the text itself.
- WinAnsiEncoding
- A single byte text encoding used by simple PDF fonts, close to Windows code page 1252.
- Zero mutation editing
- Editing a PDF so that everything you didn't change stays exactly as it was in the original file.
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