Can the Gmail iOS app sign PDFs?

No. It has no markup or signing tools, so the attachment has to be opened elsewhere — Files, or a dedicated PDF app — and then attached back to a reply.

Should I reply or send a new email with the signed form?

Reply on the original thread. Senders usually track returns against their thread, and a new message with a different subject is the most common way a returned form gets missed.

How to Sign a PDF From Gmail on an iPhone

Gmail's iOS app does not sign PDFs, which is why this feels harder than it should. The two routes that work, and which one to use.

1 min read

Apple Mail has signing built in, so a PDF attachment there is a long-press away from being signed. The Gmail app does not, which is why people who use Gmail on an iPhone assume signing on a phone is harder than it is. The attachment just needs to leave Gmail first.

Route one: out to Files, back to Gmail

Open the message in Gmail and tap the attachment.
Tap the share icon and choose Save to Files. Put it somewhere you will find again.
Open it from Files, fill and sign it there.
Share the finished file back into a Gmail reply.

This is the route to use when you want to keep the signed copy, which for anything contractual you do.

Route two: share straight into a signing app

From the same share sheet, send the attachment directly to a PDF app rather than to Files. Fewer steps, and it skips the question of where the file landed — but it also means the only copy is inside that app until you export it.

Why the reply matters more than people think

Attach the signed PDF to a reply on the original thread rather than a new message. The sender is usually tracking a list of who has returned a form, and a reply keeps your document attached to their thread. A fresh email with the subject "signed" is how forms get lost.

Name the file usefully too. "scan_0001.pdf" tells the recipient nothing; "lastname-w9-signed.pdf" tells them everything, and costs you five seconds.

One thing to check before sending

Open the attachment from the sent message and look at it. Not the copy in your app — the one that actually left. It is the only way to catch an export that dropped a field, and it takes ten seconds.

Questions

Can the Gmail iOS app sign PDFs?: No. It has no markup or signing tools, so the attachment has to be opened elsewhere — Files, or a dedicated PDF app — and then attached back to a reply.
Should I reply or send a new email with the signed form?: Reply on the original thread. Senders usually track returns against their thread, and a new message with a different subject is the most common way a returned form gets missed.