Text layer

The text layer is the part of a PDF that holds real, selectable characters, as opposed to a picture of text. When you can click and drag to highlight words, search for a phrase, or copy a sentence out of a PDF, you are interacting with its text layer.

Not every PDF has one. A document exported from a word processor has a rich text layer from the start. A scanned page, by contrast, is just an image, there is no text underneath, which is why you cannot search or select it. OCR is what fixes this: it reads the image and builds an invisible text layer beneath the visual page.

The text layer is also what makes a PDF accessible to screen readers and useful to search engines. If you need to find, extract, or read content programmatically, the text layer is what you are reaching for.