You get an email with a PDF contract attached. Someone needs your signature on it by the end of the day. You are not at your desk. You do not have a printer, a scanner, or a stylus. All you have is your phone. Can you sign that document using only the tools already built into your device, without downloading a single app?
The answer is yes on both iPhone and Android. Both platforms include free, pre-installed markup tools that can add a signature to any PDF in under a minute. You do not need Adobe Acrobat, you do not need a third-party signing app, and you do not need to print anything. The built-in tools are basic, but for a clean, legally presentable signature, they get the job done.
How to Sign a PDF on an iPhone Using Only the Built-In Markup Tools
Every iPhone ships with a feature called Markup, built directly into the Files app, the Mail app, and the screenshot editor. To sign a PDF, open the file in the Files app. If the PDF arrived as an email attachment, tap the attachment to open it, then tap the Share icon and choose Save to Files. Navigate to where you saved it and tap the file to open it.
Once the PDF is open in Files, tap the Markup icon, which looks like a pen tip inside a circle, located in the top-right corner of the screen. The Markup toolbar appears at the bottom. Tap the plus button, which looks like a circle with a plus sign, and choose Signature from the menu. If you have never created a signature before, you will see an option to add one. Use your finger to draw your signature on the screen. Take your time. A slow, deliberate stroke produces a better result than a rushed scribble. Tap Done to save the signature.
After saving, your signature appears as a movable, resizable object on the PDF page. Drag it to the signature line. Pinch to resize it if it is too large or too small. You can add additional signatures if the document requires multiple places to sign. When you are finished, tap Done. The signed PDF is saved automatically in place. You can now share it via email, Messages, or any other app directly from the Files app. The entire process, opening the file, drawing the signature, placing it, and sending it back, takes less than two minutes once you have done it once.
How to Sign a PDF on an Android Phone Using Built-In Tools
Android does not have a single universal PDF markup tool built into every device the way iOS does, because Android manufacturers customize their software. However, Google Files, which comes pre-installed on most Android phones and is available for free on the Play Store for all others, includes PDF viewing and basic markup. Open the PDF in Google Files by navigating to the Downloads folder or wherever the file is stored. Tap the file to open it in the viewer.
At the bottom of the screen, look for an Edit or Markup icon. If Google Files does not show editing options, a second built-in approach works across all Android devices: open the PDF in Google Drive. Google Drive includes a PDF annotation mode. Tap the pen icon in the bottom-right corner after opening the file. From the annotation toolbar, you can select the pen tool and draw your signature directly on the document with your finger. While this is not as polished as Apple's dedicated Signature tool, it produces the same result: a signed PDF saved back to your device.
Samsung Galaxy devices include an additional option through Samsung Notes, which can import PDFs and let you write on them with the S Pen or your finger. Huawei, Xiaomi, and other manufacturers each ship their own file manager or note app with similar capabilities. The common thread across all Android devices is that the functionality exists in the pre-installed apps. You do not need to visit the Play Store.
What the Built-In Tools Do Well and Where They Fall Short
The built-in Sign PDF tools on both iPhone and Android excel at the core task: placing a visual representation of your signature onto a PDF page. For everyday documents, permission slips, vendor agreements, internal memos, lease renewals, the signature you produce with the built-in tools looks professional and serves its purpose. The process is fast, free, and requires no accounts or installations.
Where the built-in tools fall short is in security and advanced signing features. A signature drawn with Markup or Google Drive is a static image placed on top of the PDF. It is not a cryptographically verifiable digital signature. It provides no audit trail, no tamper-evident seal, and no certificate-based identity verification. For high-stakes documents, such as real estate contracts, notarized forms, or regulated financial agreements, a visual signature alone may not meet the legal or compliance requirements of the receiving party.
The built-in tools also cannot handle multiple signatures in sequence. If a document needs to be signed by you, then by your manager, then by a client, each person would need to open the file on their own device, add their signature, and pass it to the next person. There is no workflow management, no status tracking, and no automatic notification that the next person should sign. For simple one-person signatures, this limitation is irrelevant. For multi-party signing workflows, a dedicated e-signature platform handles the orchestration that built-in tools cannot.
Tips for a Clean Signature Using Only Your Finger
Drawing a signature with your fingertip on a glass screen is not the most natural writing experience. A few techniques produce noticeably better results. First, rotate your phone to landscape mode before drawing. The wider canvas gives you more horizontal space and makes it easier to produce a signature that looks proportional rather than cramped. Your finger takes up more room than a pen tip, so use the full width of the screen.
Second, rest the side of your hand on the screen lightly if your device supports palm rejection. Not all phones handle this well, but most modern iPhones and flagship Android devices do. If palm rejection is inconsistent on your device, hold the phone with one hand and sign with the index finger of the other hand, keeping the rest of your fingers away from the screen.
Third, do not try to replicate your pen-and-paper signature exactly. Your finger moves differently than a pen on paper. Aim for something recognizable as your mark rather than a perfect reproduction. Most recipients will compare it to your driver's license signature, not a forensic analysis. A signature that looks plausibly like yours is sufficient for the vast majority of non-notarized documents.
When You Should Use a Browser-Based Signing Tool Instead
If you need a signature that looks more polished than a finger-drawn scribble, or if you sign documents on your phone regularly, a browser-based PDF Tools option provides a better experience without requiring an app download. WukongPDF's sign tool lets you type your name and convert it to a cursive-style signature, upload an image of your handwritten signature, or draw one using a larger canvas optimized for mouse and touch input. The output is a clean, properly scaled signature placed on the PDF, and the tool generates the signed PDF in seconds right in your phone's browser.
The workflow is similar to the built-in tools: open the website, upload the PDF, place the signature, and download the signed file. The difference is in the quality of the result. A typed-then-converted cursive signature looks more professional than a finger drawing. A pre-saved signature image stored on your phone produces a consistent signature every time without needing to redraw it. For anyone who signs documents on their phone more than once a week, these small quality differences compound into a meaningfully better experience.
The built-in tools are always there when you need a signature in a hurry and do not want to download anything. They work right now, on the device already in your hand. Knowing they exist means one less thing to install and one less account to create. For the quick signature on the go, they are exactly the right tool for the job.