What is a PDF file?

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.

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.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

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.

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

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.

PDF is not limited to one page, which matters when converting to something that is: one conversion produces one file holding a single page — the first one, unless the page is something the converter lets you pick.

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.

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.

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.

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.

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".

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.

A PDF is a list of drawing instructions: put this glyph of this font at this coordinate, stroke this line, place this image in this rectangle. There are no paragraphs in it, no headings, no sentences — only marks positioned on a fixed canvas.

That is why it looks identical everywhere, and it is the source of nearly every difficulty with it. A word processor file knows it contains a paragraph; a PDF knows it contains sixty-three characters at sixty-three positions that happen to line up. Everything below follows from that distinction.

A born-digital PDF was written by software — exported from Word, printed to PDF, generated by an invoicing system. It contains real text as characters, and copying, searching and extracting all work.

A scanned PDF is a photograph of paper. It contains one image per page and no text whatsoever, so searching finds nothing and selecting produces nothing. They look the same in a viewer and behave completely differently, and the first question about any PDF problem is which of the two you are holding. Try selecting a word: if the cursor sweeps across the page without highlighting anything, it is a scan.

Making a scanned PDF searchable means running optical character recognition, which reads the image and writes invisible text behind it aligned to what it saw. The picture you look at is unchanged; a searchable layer now sits underneath.

That layer is a best guess. Clean printed text at a decent resolution comes out very accurate; a faxed contract, a curled book spine, a handwritten note or a low-resolution scan produce errors that are silent — the wrong word is simply there, spelled confidently. Searchable is not the same as correct, which matters when the document is being relied on.

Since the file holds positioned marks rather than structure, producing an editable document means inferring the structure back: deciding which marks form a paragraph, which line is a heading, where a table’s cells are, which column comes first on a two-column page.

Good tools do this well on simple layouts and struggle on complex ones. Expect a clean single-column report to come out close to right, and expect a magazine spread, a form or a densely formatted invoice to come out approximately. That is inherent in the format rather than a shortcoming of any particular converter — the information being recovered was never written down.

A PDF normally carries the fonts it uses inside the file, often subsetted to just the characters that appear. That is precisely why a PDF looks identical on a machine that has never seen the typeface, where a DOCX would substitute something and reflow.

It is also why PDFs are larger than the text in them suggests, and why a document using several fonts costs more than one using two. Where a font is not embedded — some older files and some generators — the viewer substitutes, and the layout can shift in exactly the way the format exists to prevent.

Drawing a black rectangle over a name adds a rectangle. The name is still in the file underneath it, and copying the text or examining the content stream retrieves it immediately. This has produced a long series of public embarrassments involving court filings and released documents.

Proper redaction removes the underlying content and then draws the mark. Any tool offering a redaction feature does this; a drawing tool does not. The reliable test is to select and copy over the blacked-out area — if the text arrives on the clipboard, nothing has been removed.

An owner password sets permissions — no printing, no copying, no editing. The bytes are encrypted, but against an empty user password, so every viewer opens the file without asking and then decides for itself whether to honour the flags. It is a request, not a lock.

A user password encrypts the contents, and without it the document genuinely cannot be read. Modern PDFs use AES and are properly protected; very old ones used 40-bit encryption that is broken. If a PDF is protecting something that matters, it is worth knowing which of the two you have applied.

PDF/A is a restricted profile for long-term preservation. It requires every font to be embedded, forbids JavaScript, external references, encryption and anything else whose meaning depends on the world outside the file.

The point is self-containment: a PDF/A can be rendered correctly in fifty years with no access to anything else. Government archives, courts and libraries specify it for that reason, and it is why a submission portal sometimes rejects an ordinary PDF that opens perfectly well.

Author, the software that produced it, creation and modification dates, and frequently the title from the source document — which is often a filename nobody meant to publish. A PDF exported from Word inherits the Word document’s properties, so the author name travels unless it is cleared deliberately.

Converting to PDF is the reliable way to leave a source document’s hidden content behind — unaccepted tracked changes, comments and hidden text do not appear, because the format writes only what is drawn. It is not a way to leave the metadata behind, and those two facts are easy to conflate.

Think about what people convert. Contracts, medical letters, bank statements, court documents, tax returns, identity papers. Handing all of that to a website in exchange for a download link is a poor trade that is made routinely because there is no obvious alternative.

Reading a PDF runs inside the browser tab — pages out as images, text out of a scan — and so does building one from pictures. Nothing is transmitted on those, and the free ceiling is 100 MB per file. Turning a Word or OpenDocument file into a PDF is the exception and the page has to say so: LibreOffice does that work on a server, under a 25 MB limit, because no browser lays out an office document.