PDF

What is a PDF file?

The format that looks the same everywhere. What you send when the layout has to survive the trip.

What PDF is

PDF is a document format, describing pages rather than pixels. It can hold both pixels and drawn shapes on the same page. It is used for handing a finished file over, print and archiving.

The extension is .pdf, and the full name is Portable Document Format. Both matter less than what the file can hold, which is what the rest of this page is about.

Where PDF came from

Adobe published it in 1993. The specification is ISO 32000-2.

A format that has been readable for that long is a format worth trusting with something you want back in ten years.

The specification is public

Anyone can implement it without asking permission or paying a licence, which is why it turns up in so many programs and why files written twenty years ago still open.

Nothing is thrown away

PDF stores its content exactly. Saving it again changes nothing, so it can be opened, edited and re-saved as often as you like without accumulating damage — which is what makes it a working format rather than a delivery one.

It can be transparent

PDF carries an alpha channel, so a logo keeps its soft edges over whatever sits behind it instead of arriving in a white box.

Colour it can describe

It can work in RGB, CMYK, the model a printing press needs and greyscale.

CMYK is the one that decides whether a printer accepts the file, and it is the thing most often lost converting to a screen format.

It can hold several pages in one file

A PDF file is not limited to one page, which matters when converting to something that is: a target that holds one image produces one file per page, and the numbering it chooses is worth checking before you send it on.

It keeps the layers

PDF stores the parts of an image separately rather than flattened together, which is what makes it a working file. Almost every format it converts into flattens them, and the flattening is not reversible — keep the layered original.

What it does to protect itself

PDF carries optional encryption.

An encrypted file has to be unlocked before anything can convert it, here or anywhere else — a password is not something a converter can work around, and one that claimed to could not be trusted with the file either.

What opens PDF

Adobe Acrobat, Preview and LibreOffice Draw read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

Opening it in a browser

Every current browser reads it.

That makes it a safe thing to put on a page or attach to a message without wondering what the other end has installed.

What it can carry besides the content

PDF can hold XMP and document properties.

Whether any of it survives a conversion depends entirely on the target, and the honest answer is usually "some of it".

It is a delivery format

PDF is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.

The facts, in one place

Identifiers and provenance for the PDF format.
Extension.pdf
Media typeapplication/pdf
Published byAdobe
First published1993
SpecificationISO 32000-2

Converting PDF to something else

Converting something else to PDF

Formats worth considering instead of PDF

Where these figures come from

The claims on this page are checkable, and these are the documents that settle them.