How do I sign a document on a Mac or Windows PC?

Built-in options

Mac: Preview

Preview opens PDFs by default on macOS and has had a built-in signature tool for years. Open the document, click the Markup toolbar icon, a pen tip in a circle, near the top of the window, then click the signature icon inside the Markup toolbar. You get three creation options: draw it on your trackpad with your finger or a stylus, hold a signed piece of paper up to your Mac's built-in camera and let it capture the signature automatically, cleaner than it sounds, or reuse a signature you've already saved from an earlier document. Drag it into place on the page, resize as needed, and save. No download required, this has been standard on every recent version of macOS.

Windows: no true equivalent, but close

Windows doesn't have a comparable built-in signing tool in its default PDF handling. Microsoft Edge's built-in PDF reader includes a "Draw" markup tool, letting you scribble freehand anywhere on a page with a mouse, trackpad or touchscreen, which technically works as a makeshift signature but has no dedicated signature feature, nothing saved for reuse, nothing styled specifically for signing documents. If you only need to sign something once and don't mind a rougher result, this works in a pinch with zero setup.

Free tools

For a more reliable, repeatable signing experience on Windows, the free Adobe Acrobat Reader app is the standard answer. Install it once, and its Fill & Sign tool lets you type, draw, or upload an image of your signature and place it anywhere on a PDF, saving your signature for reuse on future documents the same way Preview does on Mac. According to Adobe's own description, the free tool is built for "simple workflows that require you to add your own signature, initial a page, or request a signature from one or a few parties," which is exactly the self-signing scenario this covers. The same free Acrobat Reader app also works on Mac if you'd rather use one consistent tool across both platforms, though most Mac users find Preview's built-in option is simpler since it needs no install at all.

A quick note on Word specifically: if your document is a Word file rather than a PDF, Word has its own signature options, a Signature Line field (built around a digital certificate) and the ability to insert an image or draw on the Draw tab. My guide to signing in Word and Google Docs covers that in full, though converting to PDF first and using Preview or Acrobat Reader is often simpler for a one-off signature.

Move to an e-sign tool when you need proof

Both Preview and free Acrobat Reader do the same fundamental thing: place an image of your signature onto a document. That's completely fine when you're the only one signing. What neither gives you is a real audit trail, the timestamp, IP address, and tamper-evident hash that proves exactly who signed a specific version of a document and when. That gap matters in two situations specifically: when you're sending a document to someone else to sign rather than signing it yourself, and when real money or legal obligation is riding on the document in a way that could get disputed later.

For those situations, upload the finished document to a dedicated e-signature tool instead of signing inside Preview or Acrobat Reader. My guide to getting a document signed online covers the free and low-cost options, several of which cost nothing for occasional use and take only a few minutes more than the built-in desktop tools.

Common mistakes signing on desktop

• Using Edge's Draw tool for anything beyond a one-off, low-stakes signature. It has no saved-signature feature and produces a visibly rougher result than a dedicated tool, fine once, tedious and unpolished if you're signing things regularly.
• Not saving your signature for reuse. Both Preview and Acrobat Reader let you save a signature once and reuse it every time afterward. Redrawing or re-uploading it on every document wastes time for no reason. Options for the signature itself are in creating an electronic signature.
• Signing a document that's still a shared, editable file rather than a locked PDF. If you're signing a Word doc directly rather than converting it to PDF first, anyone with the file can still select and move your inserted signature afterward with no visible trace. Converting to PDF first makes casual tampering at least slightly harder.
• Assuming a built-in tool is fine for collecting someone else's signature. This is the single most common mix-up: these desktop tools are built for signing your own documents, not for sending something to a customer and getting defensible proof they signed it.

A few practical differences worth knowing between the two platforms

Beyond the headline built-in-versus-install difference, a couple of smaller details are worth knowing if you switch between Mac and Windows regularly. A signature you save inside Preview on one Mac generally doesn't carry over automatically to a different Mac or to Acrobat Reader, each tool and each installation tends to keep its own saved signature separately. If you regularly sign from more than one machine, uploading the same clean image file as your signature source on each device, rather than redrawing a fresh one by hand every time, keeps the result looking consistent no matter which computer you're on. It's also worth checking, if you photograph a paper signature for the upload method on either platform, that the cropped image has a clean, even background before saving it as your go-to file, a shadow or an off-white tint around the signature is easy to miss until it's sitting next to crisp black text on a finished document.

A quick worked example

Say a vendor emails you a one-page NDA that needs your signature before a call this afternoon. On a Mac: open it in Preview (it'll likely open there automatically), click Markup, click the signature icon, pick your saved signature or draw a fresh one, drag it into place, save, and reply with the signed file attached, all in under a minute if you've signed before. On Windows with Acrobat Reader already installed: open the file in Acrobat Reader, click Fill & Sign, place your saved signature, save, and attach to your reply. Either way, for a document only you're signing, this beats printing, signing by hand, and scanning it back in by a wide margin. The time difference compounds too, across a handful of documents a week, the minutes saved by not touching a printer at all add up to a genuinely meaningful chunk of a working day over a month. Signing straight from your inbox is explained in how to sign from an email.

How I'd approach this

I use Preview's built-in signature tool constantly for my own paperwork and never think twice about it, there's no reason to involve anything more complex for signing something only I'm signing. The line I'd draw for a client is simple: if you're the one signing, Preview or Acrobat Reader is genuinely enough. If you're the one collecting someone else's signature, especially on anything with real money or obligation attached, that's a different job entirely, and it's worth the small extra step of using a proper e-signature tool instead. Get in touch if you want your whole document and signing flow built properly rather than piecemeal.