PDF glossary: the terms you actually meet in real documents
A PDF looks like a printed page, but underneath it is a file format with its own vocabulary. Words like metadata, flattening, rasterization and linearization turn up in printer settings, support threads and file-size reports — and getting one of them wrong usually means buying the wrong kind of tool, or believing a document is safe to share when it is not.
This glossary explains the PDF terms you meet while working on real documents, in the plainest language we can manage. It is written from the inside: every definition below is tied to something the tools on this site actually do, or to something they honestly do not do. Where a task needs software PDF Forge does not offer — optical character recognition, true redaction, editing text that is already inside a PDF — we say so rather than quietly implying otherwise.
Why the words matter
Most PDF problems are vocabulary problems in disguise. Someone asks to make a file smaller, and what they actually need is to know that "compressing" a PDF usually re-renders every page as a picture — which shrinks a scan dramatically and turns a typed report into something no longer searchable. Someone else wants a name hidden before sending a contract on, and the black rectangle they draw in a viewer hides it from the eye while leaving the original words in the file. Knowing that rasterization is not compression, and that hiding is not deleting, is most of the battle.
The same goes for scanning. A scanned PDF is a stack of photographs of paper, so it carries page images and no text. That single fact explains why a search inside it returns nothing, why extracting text from it produces an empty file, and why a copy-and-paste from it fails — three confusing symptoms with one dull cause.
How to use this glossary
Terms are listed alphabetically. Read a definition, then follow the link underneath it to the tool that handles that job, or browse all the tools. For direct answers to common questions there is PDF questions answered, and the step-by-step guides cover each task in full.
PDF terms, A to Z
- DPI (dots per inch)Conversion
- How finely an image or a rasterized page is sampled: 72 DPI is screen resolution, while 150 to 300 DPI is print resolution. Exporting PDF pages as images at a higher DPI gives a sharper — and considerably larger — picture.
- In practice Exporting one page of a brochure as a 200 DPI PNG for a slide deck.
- PDF to Image
- Digital signature vs signature imageForms & signing
- A digital signature is cryptographic and travels with a certificate proving the document has not changed since it was signed. A signature image is simply a picture of your name placed on the page — convincing to look at, but it proves nothing about who signed or whether the file was altered afterwards.
- In practice Typing your name onto a PDF creates a signature image; a notary service issues a digital signature.
- Sign PDF
- Encryption and password protectionSecurity & privacy
- A PDF can require a password just to open, or carry a password that only restricts editing, copying and printing. Tools that rebuild page content need an unlocked file, so an encrypted document has to be unlocked first before it can be merged, split, resized or compressed.
- In practice A bank statement that asks for your date of birth before it opens is encrypted.
- Flattened form vs editable formForms & signing
- An editable form keeps its fields so answers can be changed, corrected or reused later; a flattened form keeps only the appearance of the answers. Keep a form editable while you are still filling it in, and flatten a copy when it is ready to send.
- In practice Fill a rental application today, flatten the final version the day you send it.
- Fill PDF
- FlatteningForms & signing
- Turning fillable form fields and annotations into permanent page content. After flattening, the values stay visible but can no longer be typed into or changed — normally the last step before sending a completed form.
- In practice Filling a tax form and flattening it so the recipient sees your answers but cannot edit them.
- Flatten PDF
- Form fieldForms & signing
- An interactive box embedded in a PDF — a text box, checkbox, radio button, dropdown or signature area — that accepts typed or clicked input. A paper form that was scanned has no form fields at all; only a PDF built as a form does.
- In practice Clicking a checkbox on a PDF application uses a form field; typing over a line on a scan does not.
- Fill PDF
- Linearization (Fast Web View)File size
- A way of saving a PDF so that the first page can be displayed before the rest of the file has downloaded. The document itself is unchanged — same pages, same quality — and the file is usually a little larger.
- In practice A 200-page manual opens on page 1 immediately because the publisher saved it linearized.
- Lossy vs lossless compressionFile size
- Lossy compression discards some image detail to save space; lossless compression rearranges the data without throwing anything away. Both shrink a PDF, but lossy compression is what makes a 20 MB scan emailable and what makes small type look grey and fuzzy.
- In practice Choosing the strongest setting in a PDF compressor is choosing lossy compression.
- Compress PDF
- MetadataSecurity & privacy
- The information a PDF carries about itself: title, author, subject, keywords, creator and producer. It is written by whatever program made the file and survives copying, merging and emailing, which is why a document can name its author and their software long after nothing on the visible page does.
- In practice A résumé exported from Word often still names its author in the file's metadata.
- Clean PDF Metadata
- OCR (optical character recognition)Conversion
- Software that reads the shapes on a scanned page, works out which letters they form, and turns a picture of text into real, selectable text. OCR is a separate step from opening or converting a PDF: a scanned PDF can look perfectly readable on screen and contain no text whatsoever.
- In practice Searching a scanned contract for a name returns nothing until that file has been through OCR.
- PDF (Portable Document Format)Basics
- A file format that fixes a document's layout — text, images and fonts — so it looks the same on every device and printer. The useful mental model is an electronic printout: faithful to look at, and not designed to be rewritten.
- In practice A signed contract, a boarding pass and a scanned receipt are all usually PDFs.
- Page rangePages & layout
- The list of pages an operation applies to, written as single numbers or spans, such as 1-3,7,10. Almost every PDF tool that changes a document lets you act on a page range instead of the whole file.
- In practice Splitting a report into chapters with ranges like 1-12 and 13-40.
- Split PDF
- Page rotationPages & layout
- A stored instruction telling a viewer to turn a page a quarter, half, or three-quarters turn when it draws it. That instruction is why a scan can open sideways, and changing it is lossless — only the orientation changes, never the content.
- In practice A scan displaying upside down is fixed by rotating it 180°, with no need to feed the paper through again.
- Rotate PDF
- Page size (A4, Letter, Legal)Pages & layout
- The physical dimensions of the sheet a page is laid out for. A4 and US Letter are close but not identical, which is why a document designed on one can print with clipped edges on the other, and why documents assembled from files of several origins end up with pages of several sizes.
- In practice Merging a Letter-sized invoice with A4 receipts gives one PDF with two different page sizes.
- Resize PDF Pages
- RasterizationConversion
- Rendering a page as a grid of pixels rather than as text and vectors. Once a page is rasterized its text is no longer selectable or searchable and it will not scale cleanly, but the page becomes far easier to compress because pictures compress well.
- In practice Exporting a page as a JPG rasterizes it, and so does compressing a PDF by re-rendering each page.
- PDF to JPG
- RedactionSecurity & privacy
- The permanent removal of content from a document before it is shared. Drawing a black rectangle over text is not redaction: in most viewers the words are still in the file, merely covered, and a text extractor will happily copy them out. True redaction deletes the underlying content and re-saves the page.
- In practice Black boxes over a name in a court filing have repeatedly leaked the original text.
- Selectable text vs scanned imageBasics
- Whether a page holds characters a computer can read, or a photograph of characters that only a human can. Selectable text can be searched, copied out and edited; a scanned image cannot, which is why the two files of the same document behave so differently.
- In practice Dragging to highlight a sentence works on a digital invoice and does nothing on a photographed one.
- PDF to Text
- WatermarkEditing
- Faint text or artwork stamped across a page, usually to mark status such as DRAFT or CONFIDENTIAL, or to identify where a copy came from. A text watermark is real page content, so it prints and appears in every viewer rather than sitting on a separate layer.
- In practice Sending a draft to a client with DRAFT marked across every page.
- Watermark PDF
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