Tap the Share icon (the square with the arrow). Scroll down and choose Print. On the preview screen, use two fingers to pinch outward. This instantly converts the preview into a PDF. Tap the Share ...
There's a lot of good stuff included in iOS 26, which makes it easy to miss one of them right in front of your eyes.
With Apple's built-in tools and apps, you can sign documents on your iPhone quickly, whether they're PDFs, legal contracts, ...