What is a PDF and what can you do with one?

What does PDF stand for?

PDF stands for Portable Document Format. Adobe released it in 1993 so that a document could be sent from one computer to another and still look right, even if the receiving computer had different fonts, a different operating system or no copy of the program that made the file. Since 2008 the format has been an open ISO standard (ISO 32000), which is why so many programs, not just Adobe's, can create and read PDFs.

A PDF document is any file that ends in .pdf. It can be one page or thousands, and it can hold text, pictures, vector drawings, links, form fields, bookmarks and attachments. Bank statements, tax forms, leases, boarding passes, user manuals and scanned letters are all commonly delivered as PDFs.

Why does a PDF look the same everywhere?

A Word document stores a stream of text plus formatting instructions and lets the program on your computer decide where each line breaks. Change the fonts installed or the printer selected, and the pages shift. A PDF works the other way around. It stores the finished layout: every letter, line and picture has a fixed position on a page of a fixed size, measured in points. The fonts used are usually embedded inside the file, so the viewer does not need to have them installed.

That is why a PDF is the safe choice when the layout matters: a resume, an invoice, a signed contract or a form that must print exactly as designed. It is also why changing the words inside a PDF later takes a dedicated tool such as Edit PDF rather than a normal word processor. The text is there, but it is pinned in place rather than flowing.

PDF vs Word vs image: which one should you use?

Each of the three formats stores a document differently, and each is good at something the others are not.

A good rule: write in Word, send as PDF. If someone sends you a photo of a document, turning it into a PDF with the text recognized makes it far easier to search and file.

• Word (.docx): best while you are still writing. Text reflows, spell check works, and anyone with Word, Google Docs or LibreOffice can keep editing. The layout can shift from one computer to another.
• PDF: best when the document is finished and needs to be shared, printed or signed. The layout is locked, the file opens on every device, and the text stays searchable and selectable in most cases.
• Image (JPG or PNG): best for a single picture or a quick screenshot. Pages are just pixels, so there is no text to search or copy, and a multi-page document becomes several separate files.

What is a PDF reader?

A PDF reader is any program that can display a PDF on screen and print it. Adobe Acrobat Reader is the best known, and it is free, but you probably do not need to install anything. Every major web browser (Chrome, Edge, Firefox, Safari) opens PDFs on its own, and Windows, macOS, iOS and Android all ship with a built-in viewer.

Readers vary in what they add on top of viewing. Most let you zoom, search, rotate pages, fill form fields and add simple comments or highlights. Some, including Acrobat Reader, Edge and Apple's Preview, can also draw a signature. What none of them do without a paid upgrade is rewrite the existing text of the page.

How to open a PDF on Windows, Mac, iPhone and Android

In most cases a double click or a tap is enough. Here is what happens on each platform, and what to do if the file opens in the wrong app.

• Windows 10 and 11: PDFs open in Microsoft Edge by default. To use a different program, right click the file, choose Open with, pick the app and check Always use this app.
• Mac: PDFs open in Preview. Preview can also rotate pages, fill forms, add a signature and export a smaller copy. To change the default app, select the file, press Command-I and set Open with.
• iPhone and iPad: a PDF received in Mail or Messages opens with a tap. Tap the Share icon and choose Save to Files to keep it. The Files app previews PDFs and offers a Markup pen for notes and signatures.
• Android: tap the file in Downloads, Gmail or Google Drive. Most phones use Google's built-in viewer or the Drive PDF viewer. If nothing is set, the free Adobe Acrobat Reader app from the Play Store works.
• Any browser: drag a PDF into a Chrome, Edge or Firefox window and it opens as a tab. From there you can print it, save it or search its text with Ctrl-F or Command-F.

What can you do with a PDF, and what can't you?

Out of the box, a PDF is built for reading. You can zoom, search the text, copy a passage, print it, jump to bookmarks and follow links. If the sender added form fields, you can type into them and save. Most viewers also let you highlight, add notes and draw on the page without changing the underlying content.

Some things are harder than people expect. Freely rewriting paragraphs is not what the format was built for, because text does not reflow. Pulling a table into a spreadsheet needs a conversion step. And a PDF that was made by scanning paper is really a picture of each page: you cannot search or copy its text until you run optical character recognition (OCR) on it, which adds a hidden text layer behind the image.

What format is a PDF file inside?

Underneath, a PDF is a list of objects: pages, fonts, images, text-drawing commands and metadata, wrapped in a structure that a viewer can read without loading the whole file. That is why a 500-page manual opens on page one instantly. Text is normally stored as real characters together with the font, which keeps the file small and searchable. Photos are stored as compressed images, and logos or charts are often stored as vector shapes that stay sharp at any zoom.

It helps to know which kind of PDF you have. A text-based PDF, exported from Word, a browser or an accounting program, has selectable text and a small file size. A scanned PDF holds one large image per page, so it is big, it cannot be searched, and the text looks fine but cannot be copied. A PDF/A file is a variant locked down for long-term archiving. An encrypted PDF needs a password to open, or restricts printing and copying.

Common PDF tasks and the tools that handle them

Most everyday PDF jobs take a minute in the browser. DocGopher's tools are free, need no account, add no watermark, accept files up to 300 MB and delete your file from the server after 30 minutes. Many of these jobs can also be done with software you already have, as noted below.

• Make a PDF smaller for email or an upload limit: Compress PDF. On a Mac, Preview's Export with the Reduce File Size filter does a rougher version of the same job.
• Combine several PDFs into one file, in the order you want: Merge PDF. Preview on a Mac can do this by dragging thumbnails from one PDF into the sidebar of another.
• Get the text back into an editable document: PDF to Word. Word on Windows and Google Docs can also open a PDF, with mixed results on complex layouts.
• Fix a typo, add a line of text, white out a field or highlight a passage: Edit PDF, linked above. Preview and Edge handle the highlighting and added text, but not changes to existing text.
• Add your signature to a contract or form: Sign PDF. Preview on a Mac and Markup on an iPhone can also place a drawn signature.
• Lock a PDF so it opens only with a password: Protect PDF. Word can also set a password when you Save As PDF, under Options.