The vocabulary of text extraction, defined plainly.

PDF text layer

The grid of character codes and positions drawn into a PDF page, separate from what the page image shows. No text layer means nothing to extract.

Reading order

The sequence a document's text should be read in, left to right and top to bottom within each column. Extraction can recover it correctly or scramble it.

OCR (optical character recognition)

Reading the shapes of characters in a picture of text and outputting real characters. It is the only route into a page that has no text layer at all.

Searchable PDF

A scanned page with an invisible OCR text layer added behind the image, so the page looks unchanged but its text becomes selectable and extractable.

Font subsetting

Embedding only the glyphs a PDF actually uses instead of a whole typeface, which shrinks files but can leave out the character-to-Unicode map extraction needs.

DPI (dots per inch)

A resolution unit for a scanned image, though a file's declared DPI can lie, so OCR depends on the text's pixel height, not the tag.

Character encoding

The rulebook mapping bytes to characters, so reading a file with the wrong one can turn every character wrong, even though every byte survives intact.

Mojibake

Text decoded with the wrong character encoding: the right bytes, read by the wrong rulebook. Fully reversible by re-decoding correctly.

Byte order mark (BOM)

A few invisible bytes at the start of a text file declaring its encoding and byte order. Helpful for a reader, a stray character if a parser doesn't strip it.

Ligature

A single glyph that visually fuses two or more letters (fi, fl), left unexpanded, extracts as an unrecognized symbol instead of separate characters.

Replacement character

Marks a byte sequence a decoder couldn't turn into any real character at all. Unlike mojibake, there's nothing left to recover.

Media type (MIME type)

The standard label for a file's format, like application/pdf or image/png, trustworthy only when derived from the file's bytes, not its extension or a header.

Magic bytes

The fixed byte sequence at the start of a file that identifies its real format: %PDF for a PDF, PK for a zip. It works regardless of what the extension claims.

OOXML (Office Open XML)

The zip-of-XML format behind modern .docx/.xlsx/.pptx, where each piece of content lives in its own XML part, so a parser must know where to look.

Document metadata

Data about a file, like its type, size, page count, and whether OCR ran, as opposed to its body content. What an extraction API reports alongside the text.

Caption track

Text already stored inside or alongside a video/audio file, in a sidecar file or muxed into the container. Not the same thing as transcribing speech.

Chunking

Splitting a long document's extracted text into smaller pieces sized for an embedding model. No chunking strategy recovers a reading order extraction already destroyed.

Token

The unit an LLM actually counts and charges for, usually a word piece rather than a whole word. A character-based estimate of it is a heuristic, not the real count.