You can use the Drawing app.
Select rectangular (or other) area to save, tap the Crop button, and then “Export as” to create a new .png file. (Give things a few seconds to work; it might be slow.)
Instead, I recommend GNOME Screenshot (as opposed to PureOS’ Take Screenshot app. GNOME Screenshot might need some scaling to fit, but it offers more versatility. (You can use the Mobile Settings app’s Compositor to auto-scale it, or the gsettings method described in my demo.)
Start by choosing “Selection” as the capture area, and a time delay in seconds. Tap “Take Screenshot,” go to the area you want to capture, drag your finger across the area in a rectangle (i.e. top left corner to lower right corner, for example), and wait for the screenshot to execute. Save, discard, or try again. TIP: It probably helps to start with both the screenshot app and the target display open at the same time.