In iOS, Apple includes a built-in screenshot feature which also provides you with the ability to capture an entire webpage on your iPhone or iPad that you can then save or share with someone as a PDF document.

Don't expect this option to appear when using a third-party browser app like Firefox or Chrome, as it's only available in Safari, Apple's native web browser. With that caveat in mind, here's how the feature works.

Launch Safari browser on your ‌iPhone‌ or ‌iPad‌.
Navigate to the web page that you wish to capture.
If your device lacks a Home button, press the power button located at the top of the device and the volume up button located on the right of the device at the same time to take a screenshot. Otherwise, press the Home button and the Sleep/Wake button simultaneously to capture a screenshot.
A preview of the screenshot will pop up in the lower left of the display. Tap it to open up the Instant Markup interface. You'll have about five seconds before it disappears.
Tap the Full Page tab in the upper right corner of the Markup interface.
Tap the Crop icon at the top.
Use the frame around the webpage to select what to capture by dragging the corners with your finger, then tap Done.
Tap the Actions icon (the square with an arrow pointing out) to bring up a panel of share and save options.
From here, you can either share the captured webpage as a PDF document using the top two rows of icons, or save it somewhere (Save to Files, for example) using the Action menu options below.