What is a PDF? The format, explained
What a PDF file is, what is inside one, why it looks the same everywhere, how to open and edit it, and how PDF/A differs from a normal PDF.
A PDF (Portable Document Format) is a file that records exactly how a set of pages looks, down to every font, line and image, so that it appears the same on any phone, computer or printer. That is why forms, invoices, statements, tickets and contracts arrive as PDFs: what you see is what the sender saw.
A short history
Adobe co-founder John Warnock started the project in 1991 to let documents travel between computers without losing their layout, and Adobe released PDF in 1993 with the first version of Acrobat. In 2008 the format became an open international standard, ISO 32000-1, so anyone may write software for it. The current version, PDF 2.0, is ISO 32000-2.
What is inside a PDF
A PDF is a structured file of objects: pages, fonts, images, drawings, and the links between them.
- Text, stored as characters placed at exact positions, with the fonts needed to draw them, usually embedded so the file does not depend on what is installed on your computer.
- Vector graphics for lines, shapes and charts, which stay sharp at any zoom.
- Images, such as photos, logos and scanned pages, usually compressed as JPEG or similar.
- Optional extras: links, bookmarks, fillable form fields, comments, file attachments, passwords and permissions, and digital signatures.
Because positions are fixed, a PDF does not reflow like a web page or a Word document. That is the format’s strength for printing and its weakness for editing and small screens.
Text PDFs and scanned PDFs
Two PDFs can look identical and be completely different inside.
| Text PDF | Scanned (image) PDF | |
|---|---|---|
| Made by | Exporting from Word, Google Docs, a browser, an app | A scanner or phone camera |
| Select and copy text | Yes | No, until OCR is run |
| Search with Ctrl+F | Yes | No, until OCR is run |
| File size | Small, usually KB | Large, often MB per page |
| Zooming in | Stays sharp | Gets blurry |
To tell which you have, try to select a word. If the whole page highlights as one block, it is a scan.
The PDF family
PDF has several stricter variants, each an ISO standard for a particular job:
| Variant | Standard | Built for |
|---|---|---|
| PDF/A | ISO 19005 | Long-term archiving |
| PDF/X | ISO 15930 | Commercial printing |
| PDF/UA | ISO 14289 | Accessibility for screen readers |
| PDF/E | ISO 24517 | Engineering drawings |
What is PDF/A?
PDF/A is PDF for documents that must still open correctly decades from now. It keeps everything needed to display the file inside the file, and bans anything that might stop working:
- Required: every font embedded, colours defined with an embedded colour profile, and metadata in a standard form.
- Banned: encryption and passwords, JavaScript, audio and video, and links to fonts or content outside the file.
There are several versions. PDF/A-1 (2005) is based on PDF 1.4. PDF/A-2 (2011) adds JPEG 2000 compression, transparency and layers. PDF/A-3 (2012) allows any file to be attached, which is how electronic invoices such as Factur-X and ZUGFeRD carry their data. PDF/A-4 (2020) is based on PDF 2.0. Within a version, conformance level b (basic) guarantees the pages look right, while a (accessible) also requires tagged structure for screen readers, and u requires all text to be mapped to Unicode.
If a court, government archive or tender portal asks for PDF/A, export it that way from the start: Word has a PDF/A compliant option in its PDF export settings, and LibreOffice has an Archive (PDF/A) setting. Converting a finished PDF later is possible in Acrobat Pro, but can fail on files that use banned features.
Opening, combining and changing PDFs
- Opening: every modern browser reads PDFs, as do Preview on a Mac and the Files app on phones. You do not need Adobe Reader.
- Combining: to join several PDFs, or a mix of PDFs and photos, into one file, use Merge PDF or Images to PDF. Both run in your browser, so nothing is uploaded.
- Rearranging: pages can be removed, extracted, reordered or rotated without touching what is on them, because each page is a self-contained object.
- Editing text: possible in an editor such as Acrobat Pro, but fragile. Editing the original document and exporting again is almost always cleaner.
More PDF guides: how to sign a PDF, how to make a PDF smaller, and how to remove a password from a PDF.
Tools used in this guide
Free, and they run in your browser — nothing is uploaded. See every tool on the PDF Tools page.
Frequently asked questions
What does PDF stand for?
Portable Document Format. Portable because a PDF looks the same on any device, operating system or printer, whatever software made it.
Who owns the PDF format?
Adobe created it and released it in 1993, but it has been an open international standard since 2008, when it was published as ISO 32000-1. Anyone can build software that reads or writes PDFs without paying Adobe. The current version, PDF 2.0, is ISO 32000-2.
Is a PDF a picture or a document?
It can be either. A PDF exported from a word processor holds real text that you can select, search and copy. A scanned PDF usually holds a picture of each page, so the text cannot be selected until it is run through OCR (optical character recognition).
Why can't I edit my PDF?
PDF is designed to fix a layout, not to be edited. Text is stored as positioned characters rather than flowing paragraphs, so changing a sentence means reflowing everything by hand. The file may also carry a permissions password that blocks changes. The reliable route is to edit the original document and export a new PDF.
What is the difference between PDF and PDF/A?
PDF/A is a stricter version of PDF for long-term archiving. Every font must be embedded, colours must be defined exactly, and anything that depends on outside resources or could stop working is banned, including encryption, JavaScript, and audio and video. Courts and archives often require PDF/A.
How do I open a PDF without Adobe Reader?
Every modern web browser opens PDFs: drag the file onto a Chrome, Edge, Firefox or Safari window. Macs have Preview, and iPhones and Android phones open PDFs from the Files app or Google Drive.