What is a PDF file?

A PDF file (Portable Document Format) is a document that stores page layout, text, fonts and images in one package so it looks the same on any screen or printer. Adobe created it in 1993, and it has been an open ISO standard (ISO 32000) since 2008. You can open one in this browser viewer without installing anything.

Who made PDF and what ISO 32000 means

Adobe introduced PDF in 1993 together with the first Acrobat software. The goal was simple: a file that prints and displays exactly as the author laid it out, no matter which computer, fonts or application the reader has. For 15 years Adobe published the specification itself and let anyone write software for it.

In 2008 the format became ISO 32000-1, an international standard based on PDF 1.7. The current edition, ISO 32000-2, describes PDF 2.0. Being an ISO standard means no single company controls it: Apple Preview, Chrome, Firefox, LibreOffice and open-source engines such as pdf.js (which this viewer uses for rendering) all read the same public rules.

You will also meet specialized flavors built on top of the base standard: PDF/A for archiving, PDF/X for print production and PDF/UA for accessibility. They are ordinary PDFs with extra constraints, not separate formats.

Why a PDF looks the same everywhere

A Word document stores text and lets the application decide where lines break and pages end, which is why the same .docx can paginate differently on two machines. A PDF stores the finished result instead: each page is a list of drawing commands ("place this glyph at these coordinates, draw this line, paint this image") in a fixed page size.

Fonts are usually embedded in the file, often as a subset containing only the characters actually used. That is what keeps the layout stable. When a producer skips embedding, the reader substitutes a similar font and spacing can shift slightly; that is the most common reason a PDF "looks different" on another computer.

The flip side of fixed layout is that editing is hard. There are no paragraphs to reflow, only positioned glyphs, so changing a sentence means rewriting a page. That is why most free tools, including this viewer, add content on top of the page rather than rewriting the original text. See how to edit text in a PDF for what actually does the job.

Text PDF vs scanned PDF

Two files can look identical on screen and behave completely differently. A text PDF (exported from Word, a browser, or a design program) contains real characters: you can search it with Ctrl+F, select and copy sentences, and screen readers can read it aloud.

A scanned PDF is a photo of each page wrapped in a PDF container. Search finds nothing, selection grabs a rectangle of pixels, and the file is often ten times larger. The fix is OCR, which recognizes the characters and adds an invisible text layer. This viewer can do that in the browser with the OCR tool; the details are in what a scanned PDF is and how OCR works.

Quick test: open the file, press Ctrl+F and type a word you can see on the page. No match means there is no text layer.

What is inside a PDF file

Under the hood a PDF is a database of objects with a cross-reference table that tells the reader where each object starts. The main object types you will run into:

• Pages with content streams: the drawing commands described above, usually compressed.
• Fonts and images: embedded resources that pages refer to.
• Annotations: highlights, comments, ink strokes, stamps and links that sit in a layer above the page. Read more in PDF annotations explained.
• Form fields: interactive text boxes, checkboxes and signature fields, covered in PDF forms explained.
• Metadata: title, author, creation date, producer software. This viewer shows them under Document properties.
• Encryption: optional passwords and permission flags, explained in PDF passwords explained.

Version numbers (1.4, 1.7, 2.0) tell a reader which features may appear; almost every file produced today is 1.4 to 1.7, and any current reader handles them.

How to open, edit and convert a PDF

Open. Every modern browser renders PDFs natively, so a download link usually just opens in a tab. For tools beyond reading, drop the file on the online PDF viewer: nothing is uploaded, the page is drawn by JavaScript on your own device, and closing the tab removes it. If a file refuses to open, this guide walks through the usual causes.

Edit. In the viewer you can highlight, draw, add text and images, place a drawn signature, cover content with a whiteout box, rotate, delete and reorder pages, and append another PDF. Download writes a new file with those changes embedded. What it does not do: change the original words, fill form fields as fields, compress, or split by page range.

Convert. Converting a PDF to an editable format means reconstructing paragraphs from positioned glyphs, which is a job for Word, LibreOffice or a dedicated converter; see PDF to Word. Going the other way is easy on any system: open the document and use Print, then Save as PDF, as described in Word to PDF.

Frequently asked questions