How do I take a full page screenshot and save to Photos instead of PDF?

The full page screenshot option does not allow for saving as a photo, it only provides the option to save as a pdf. Feedback to Apple can be provided here, Feedback - iPhone - Apple

Screenshots are always saved as an image. I am not sure what you are referring screenshot as.

How to take a screenshot on iPhone 8, 7, or 6

1. Press the side button and the Home button at the same time. 
2. 
3. Quickly release both buttons.
4. After you take a screenshot, a thumbnail temporarily appears in the lower-left corner of your screen. Tap the thumbnail to open it or swipe left to dismiss it.