How to sign a document someone sent you on WhatsApp

In Latin America a large share of documents arrive on WhatsApp: the rental contract, the school letter, the bank authorization. WhatsApp doesn't let you sign inside the chat, but you can do it from the same phone without printing anything.

Open the document in the chat and choose "Save" or "Share → Save to Files" (iPhone), or "Download" (Android). If they sent it as a photo instead of a PDF, ask them to resend it as a document: photos lose quality and can't be signed properly.

On iPhone, open the PDF in Files, tap the markup icon and choose "Add Signature". On Android, open the PDF with Adobe Acrobat Reader (free) and use "Fill & Sign". Either way you sign with your finger and place the signature on the line.

Save the signed PDF and share it back in the chat as a document, not a screenshot: a screenshot lowers the resolution and leaves out the other pages.

Sending a PDF on WhatsApp and waiting for it to come back signed has problems: you don't know if it was opened, the signature is just an image and there's no record of who signed. With an electronic signature platform, instead, you send a link: the person signs on their phone and you can see in your dashboard whether they opened it and when they signed.

On Firmiu's paid plans you can share that signing link on WhatsApp straight from the dashboard, picking the contact from your address book. For security, the 6-digit verification code still reaches the signer by email: the link alone isn't enough to sign.