PDF terms explained in plain language
Instructions and PDF tools use words that are rarely explained: flatten, rasterise, text layer, permissions password. Most of them describe simple ideas. This glossary groups them by what you are trying to do, so the word you just met sits next to the ones it is usually confused with.
What is on the page
Text layer: the real characters in a PDF, which can be searched, selected and copied. A PDF made on a computer has one; a scan usually does not. Vector: shapes and text stored as outlines, which stay sharp at any zoom — typical of charts, logos and exported documents. Raster: a picture made of pixels, like a photo or a scan, which blurs when enlarged.
Embedded font: a font stored inside the PDF so the text looks the same on every device. Subset: only the characters actually used are stored, which keeps the file smaller. Metadata: information about the file itself, such as its title, author and the program that made it.
Changing a PDF
OCR (optical character recognition): reading the words in a picture of a page and adding them as a text layer. Flatten: making something that could still change — form entries, comments, layers — a permanent part of the page. Rasterise: turning a page into a single picture; it looks the same but loses its text layer and sharpness.
Redact: deleting content from the file so it is gone, usually leaving a black box where it was. Cover (or mask): drawing a box over content that is still underneath — not the same as redacting. Compress: making a file smaller, mostly by storing images more efficiently. Crop: changing the visible area of a page; the trimmed part is hidden, not deleted.
Protecting a PDF
Open password (user password): needed to open and view the file at all. Permissions password (owner password): restricts printing, copying or editing, but the file can still be opened without it. Encryption: the scrambling that makes a password-protected PDF unreadable without the password; AES-256 is the strongest standard PDF encryption.
Watermark: text or a logo across the page that labels a copy, such as "For verification only". Digital signature: a cryptographic seal tied to a certificate that shows who signed and whether the file changed afterwards — different from an image of a handwritten signature.
Pages, images and printing
Page size: the dimensions stored for each page, such as A4 (210 × 297 mm), short (8.5 × 11 in), long (8.5 × 13 in) or Legal (8.5 × 14 in). Orientation: portrait (taller than wide) or landscape (wider than tall). Rotate: turning a page by 90 degrees; a PDF can store a rotation instead of redrawing the page.
DPI or PPI (dots or pixels per inch): how much detail an image has for its printed size; about 150 is fine for screens and 300 for small print. Bookmarks (outline): the clickable table of contents in a PDF reader's side panel. Form field: a fillable box or checkbox built into a PDF form.