Traditionally, signing a PDF is an exercise in frustration. Because most desktop PDF-editing software is a digital dumpster fire, it often seems like the most straightforward solution is the manual approach — print the document, sign it on paper with a pen, then scan or photograph it and attach the resulting file to an email to return it to the sender. But that's only the easiest method if you don't consider using your phone, because there are any number of apps that'll do it all digitally, without any need for a printer, camera, dead tree, or pen.
There are several apps in the Google Play Store that you can use to sign a PDF. And a number of them do it for free, so there's no need to ever spend money on an app or a subscription. In fact, we recommend sticking with the most popular PDF viewer: Adobe Reader. Yes, you can also use Docusign or any number of similar apps, but Adobe Reader has the ability to sign, date, and fill PDF documents, and you can export signed files via email. Why use anything else?
Step 1: Download Adobe Reader and open your PDF
If you haven't already installed the app, download Adobe Reader for Android from the Play Store.
Make sure that the document you want to sign is stored somewhere Adobe Reader can find it — you can save it on your phone's internal storage from an email attachment, for example, or save it to Dropbox or Google Drive.
Start Adobe Reader and open the PDF file. Tap the "Files" tab at the bottom of the screen and then locate the document on the phone or in a cloud storage location.
Step 2: Create your signature, and sign (or fill) your PDF
Tap the Edit button (the blue icon with a pencil inside) in the lower right corner, and then, in the pop-up menu, tap "Fill & Sign."
Tap on a line that you want to sign or initial. In the pop-up above the box, tap the fountain pen icon.
You should now see a large signature field — sign it, and then tap "Done." You'll see your signature appear in the field in the PDF. You can drag the signature box by the arrows on the right edge to resize it, and grasp the box in the middle to re-position it on the screen.
You can also add a date or other text to the screen. If the keyboard isn't visible, tap the text button ("ab") in the lower left corner of the screen, and then tap in the field where you want to add text. Tap one of the text buttons in the pop-up (you can choose from among two text sizes). Type the date or text. You can also re-position this text field when you're done.
Step 3: Save or export your signed PDF
That's pretty much it. When you're done with the document, you can send a copy via email by tapping the Share button at the top right of the screen and choosing Share a Copy.
Save the PDF with your annotations by tapping the checkmark at the top left of the screen.
Optional: How to delete or replace a saved signature
In the future, Adobe Reader will remember your signature and insert it automatically when you select the signature tool. But you can replace it with a different signature. When you tap in the document and choose the signature tool, tap the signature button at the bottom of the screen. (If you can't see the toolbar with the signature icon at the bottom of the screen, it's covered by the keyboard — minimize the keyboard and try again.
You should now see your saved signature. Tap the delete button to the right and then tap "Create Signature."