PDF Glossary
Twenty-nine words that show up in print dialogs, scanner settings and support articles, each one explained the way you would explain it to a friend. New to the format itself? Start with what a PDF is, then come back for the vocabulary.
Jump to a term
Accessibility · Annotation · Bookmark · Byte, KB, MB · Compression · DPI · Encryption · File size · Flattening · Font embedding · Form field · JPG · Lossless compression · Lossy compression · Merge · Metadata · OCR · Page · Page size · Password protection · PDF · PNG · Print to PDF · Raster image · Rotation flag · Scanned PDF · Searchable text · Split · Vector graphics
The terms, A to Z
Accessibility (tagged PDF)
A tagged PDF carries an invisible outline of its own structure: this is a heading, this is a table, this is the reading order. Screen readers use those tags to present the document sensibly instead of reading a jumble of positioned text. Tags survive best when a program exports a PDF directly rather than printing one.
Annotation
A note, highlight, stamp or drawing added on top of a page rather than woven into it. Annotations live in their own layer, which is why a reader can hide them or let you delete one without disturbing the text underneath.
Bookmark (outline)
The clickable table of contents that appears in a sidebar of many PDF readers, jumping you to a chapter or section. Bookmarks are stored in the file itself and are quite different from browser bookmarks. Exporting from a word processor with proper headings is the usual way they get created.
Byte, KB, MB
Units of file size. A byte is roughly one character of plain text, a kilobyte (KB) is about a thousand bytes, and a megabyte (MB) is about a thousand kilobytes. A text-only PDF page often lands in the tens of KB; a single scanned page can easily run to several MB.
Compression
Storing the same content in fewer bytes, either by finding smarter ways to describe it or by discarding detail nobody will miss. Nearly every PDF is compressed to some degree already. Our Compress PDF tool goes further by re-encoding pages as images at a quality level you choose.
DPI and resolution
DPI means dots per inch: how finely a page was scanned or is to be printed. Resolution is the same idea expressed as a pixel count. Higher numbers mean more detail and much larger files, since doubling the DPI roughly quadruples the data. For reading on screen, 150 to 200 DPI is usually plenty.
Encryption
Scrambling a file's contents so that only someone with the right key or password can read them. In PDFs it is applied by the program that created or secured the document, and it is the mechanism behind password protection. Our tools do not encrypt or decrypt anything.
File size
How many bytes the document occupies. In PDFs it is driven overwhelmingly by images: scan resolution, color depth and embedded photographs. Fonts, metadata and page count matter far less. Why is my PDF so large covers the causes and the fixes in detail.
Flattening
Baking separate elements, such as annotations, form fields or layers, into the page itself so they become ordinary fixed content. Flattening makes a document behave predictably everywhere, at the cost of no longer being able to edit those pieces separately.
Font embedding
Storing the typefaces a document uses inside the PDF, so it renders correctly on a device that does not have those fonts installed. Embedding is what makes the format truly portable. It also adds weight, and full character sets for languages with thousands of glyphs can add a lot.
Form field
An interactive box in a PDF that you can type into, tick or select from, such as a name field or a signature line. Fields hold values separately from the page content, which is why a half-completed form can be saved and finished later, and why flattening turns the answers into plain printed text.
JPG (JPEG)
The most common photographic image format, using lossy compression to reach small file sizes. It handles photographs and scans beautifully and struggles with sharp edges, fine lines and text, where it can leave a faint haze. Most images inside PDFs are JPGs.
Lossless compression
Shrinking data without discarding any of it, so the original comes back byte for byte when the file is opened. PNG images and the text streams inside PDFs work this way. The savings are more modest than lossy methods, but nothing is degraded.
Lossy compression
Shrinking data by throwing some of it away permanently, betting that the eye will not notice. JPG is the classic example. It is how compressed PDFs get dramatically smaller, and it is one-way: re-saving at higher quality never brings the discarded detail back.
Merge
Combining several PDFs into one document, one file's pages after another's. Nothing is re-rendered, so quality is untouched and the new file weighs roughly what the originals did together. Use Merge PDF, or read the merge guide for the full walkthrough.
Metadata
Information about the document rather than in it: title, author, subject, keywords, creation and modification dates, and the software that produced the file. It is invisible on the page but travels with the file, so it is worth a look before sending a document outward.
OCR (optical character recognition)
Software that examines a picture of text and works out which characters it is looking at, turning a scanned page into text you can search and copy. Results depend on scan quality, and mistakes happen with handwriting, poor contrast and unusual fonts. None of our tools performs OCR; they work with the file exactly as it already is.
Page
The basic unit of a PDF: a fixed rectangle with its own size, rotation setting and content. Unlike a web page, it does not reflow, which is why a PDF looks the same everywhere. Tools that merge, split or rotate are simply rearranging these objects.
Page size
The physical dimensions a page describes, most often US Letter (8.5 by 11 inches) or A4 (210 by 297 mm). A PDF can mix sizes in one document, which is common after merging files from different sources and is a frequent cause of odd printing results.
Password protection
A password that either blocks a document from being opened at all or restricts what can be done with it, backed by encryption. Our tools do not add or remove passwords, and a file that demands one to open may not load here at all. Add or lift protection in the program that owns the document.
PDF (Portable Document Format)
A file format that stores finished pages: text with exact positions, embedded fonts, images and vector shapes, plus extras such as bookmarks and form fields. Adobe created it and it later became an open ISO standard, which is why every modern device can open one. See what a PDF is for the long version.
PNG
A lossless image format, best for screenshots, logos, diagrams and anything with sharp edges or transparency. PNGs of photographs are usually far larger than the JPG equivalent for no visible gain, which is one way PDFs end up bloated.
Print to PDF
Producing a PDF by choosing it as the destination in an ordinary print dialog, rather than sending pages to a printer. It works in almost any program that can print and needs nothing installed. How to save anything as a PDF covers the pattern and its limits.
Raster image (bitmap)
A picture stored as a grid of colored pixels: photographs, screenshots and scans. Enlarge one far enough and it goes soft or blocky, because there is no extra detail hiding in the file. Rasters are the heavyweight content in most PDFs.
Rotation flag
A per-page setting recording how far a page should be turned when displayed or printed: 0, 90, 180 or 270 degrees. It is a label rather than a redrawing, which is why Rotate PDF can straighten a sideways scan instantly without touching image quality.
Scanned PDF
A PDF whose pages are photographs of paper rather than real text, produced by a scanner, copier or phone camera. It looks like a document and behaves like a photo album: nothing to select, nothing to search, and a file size to match. See why PDFs get so large.
Searchable text
Text stored as real characters, so a reader's find function can locate it and you can copy it out. Text-based PDFs have it from birth; scans gain it only after OCR, and lose it again if the pages are converted back into images.
Split (extract pages)
Taking pages out of a PDF, either as one shorter document or as separate single-page files. Useful for sending only the three pages someone asked for instead of the entire sixty-page report. Our Split PDF tool does both.
Vector graphics
Artwork described by mathematics rather than pixels: lines, curves and filled shapes. Logos, charts and table rules are usually vectors. They stay perfectly sharp at any zoom or print size and take up very little space, which is the opposite of raster images.
Frequently asked questions
What is the difference between a scanned PDF and a normal PDF?
A normal PDF stores real characters, so you can search and copy the text. A scanned PDF stores a photograph of each page, so there is nothing to search or copy until optical character recognition is run on it. The distinction also explains most surprising file sizes.
Do your tools remove a PDF password?
No. Our tools neither add nor remove passwords, and a file that demands a password to open may refuse to load here at all. Removing protection is something to do in the program that applied it, with the credentials of whoever owns the document.
Does compressing a PDF lose quality?
Yes, that is how the saving is made. Our Compress PDF tool re-encodes each page as a JPEG image at the quality level you choose, so detail is discarded and any selectable text becomes part of the picture. Keep your original file and save the compressed copy under a new name.