PDF glossary: common PDF terms explained in plain English
By the fullpdftools team · Updated
Quick answer: a PDF (Portable Document Format) is a file format that records exactly where every piece of text, image and line sits on a page, so a document looks the same on every device. Most PDF jargon describes either how that page content is stored (vector, raster, embedded fonts), how it is changed (flattening, compression, OCR) or what extra information travels with it (metadata, form fields, signatures). The terms below are in alphabetical order within each group.
File basics
- PDF (Portable Document Format)
- A document format created by Adobe in 1993 and published as the open standard ISO 32000. It stores fixed pages rather than flowing text, which is why it prints and displays consistently but is harder to edit than a Word file.
- PDF/A
- An archiving version of PDF (ISO 19005) designed for long-term storage. It requires everything needed to display the file, such as fonts, to be inside it and forbids features that depend on outside resources. Courts, archives and some government portals ask for it.
- Page size (A4, Letter)
- Each PDF page has its own dimensions, measured in points (1/72 inch). A4 is 595 × 842 points (210 × 297 mm); US Letter is 612 × 792 points (8.5 × 11 in). One PDF can mix sizes.
- Metadata
- Information stored about the document rather than on its pages: title, author, creating program, dates and sometimes keywords. It can reveal names or software, so check it before publishing sensitive files.
- Password protection (encryption)
- A PDF can require a password to open it, or a separate “permissions” password that restricts printing, copying or editing. Encrypted PDFs can’t be processed by most tools until the password is removed.
How content is stored
- Embedded fonts
- The font files (or the subset of characters actually used) stored inside the PDF so text displays correctly on computers that don’t have that font installed. They are one reason a short PDF can still be a few hundred kilobytes.
- Raster image
- A picture made of pixels, like a photo or a scan. It looks blurry or blocky when enlarged beyond its resolution.
- Vector graphics
- Shapes and text described mathematically (lines, curves, fills), so they stay sharp at any zoom level. Text in a PDF exported from Word is vector content; text in a scan is not.
- Text layer
- Real, selectable characters stored in a PDF. If you can highlight and copy words, the PDF has a text layer. Scans don’t, unless OCR has added one.
- Scanned PDF
- A PDF whose pages are photos of paper. It looks like a document but, to a computer, each page is one image. You can’t search or edit the words without OCR.
Changing and converting
- Compression (lossless and lossy)
- Making a file smaller. Lossless compression stores the same data more efficiently; lossy compression, such as JPG, throws away detail you are unlikely to notice. See how to reduce PDF file size.
- DPI (dots per inch)
- How many pixels are used for each inch of page when a page is scanned or turned into an image. 150 DPI suits screens; 300 DPI suits print. See choosing DPI for PDF to JPG.
- Flattening
- Merging separate layers (annotations, form fields, signatures, cover boxes) into the page so they can no longer be moved or edited. Flattening to an image also removes any hidden text underneath.
- OCR (optical character recognition)
- Software that recognises letters in an image and adds a text layer, making a scan searchable and copyable. Accuracy depends on scan quality; 300 DPI scans work best.
- Rasterizing
- Turning a page’s vector text and graphics into a single pixel image. It is how strong compression and secure flattening work, at the cost of selectable text.
- Redaction
- Permanently removing confidential content, not just hiding it. A black box drawn over text is not redaction unless the text underneath is deleted or the page is rasterized. See how to edit a PDF safely.
Pages and structure
- Bookmarks (outline)
- The clickable table of contents shown in a viewer’s side panel. Bookmarks point to page locations and may be lost when pages are split or merged.
- Crop box and media box
- The media box is the full physical page; the crop box is the visible area. A page can carry hidden content outside its crop box.
- Page range
- A short notation for choosing pages, such as
1-3, 5, 8-(pages 1 to 3, page 5, and page 8 to the end). See how to split a PDF. - Page rotation
- An orientation setting stored for each page (0, 90, 180 or 270 degrees). Viewers can also rotate only the view, which isn’t saved; see rotating a PDF permanently.
- Tagged PDF
- A PDF with a hidden structure that marks headings, paragraphs, lists and image descriptions, so screen readers can read it in the right order. Important for accessibility.
Forms and signatures
- Fillable form (AcroForm)
- A PDF with interactive fields you can type into, tick or choose from. A “flat” form only looks like a form; to fill it, you place text on top with an editor.
- Annotation
- Anything added on top of a page’s original content: comments, highlights, drawings, stamps. Annotations can usually be moved or deleted until they are flattened.
- Digital signature
- A cryptographic seal based on a certificate that proves who signed and shows whether the file changed afterwards.
- Electronic signature
- The broader term for any electronic sign of agreement, including a drawn or typed signature. See how to sign a PDF.
- Watermark
- Text or an image placed across pages, often semi-transparent, such as DRAFT or CONFIDENTIAL. See watermark best practices.
Frequently asked questions
What does flattening a PDF mean?
It merges annotations, form fields and other layers into the page so they can no longer be edited separately. Flattening to an image also removes any text hidden underneath.
What is the difference between PDF and PDF/A?
PDF/A is a stricter version for long-term archiving. Everything needed to display it, such as fonts, must be embedded, and features that depend on outside content are not allowed.
How can I tell if a PDF is scanned?
Try to select a word. If you can’t highlight individual words, or the whole page is selected as one picture, it is a scan without a text layer.
What does OCR stand for?
Optical character recognition: software that recognises letters in an image and turns them into real, searchable text.