PDF Viewer for iOS offers various alternatives to export pages out of a PDF document.
Using the "Export" action:
Open the Document, then tap the Activity button on the toolbar and select Export. From there, you can select the configuration you want to export.
Using the Document Editor:
Open the Document, then tap the Thumbnails button on the toolbar and tap the newly shown "Edit" button. Once you've selected the pages you'd like to export, press the "Export" button on the toolbar.
Exporting via Drag & Drop to other applications
With a valid PDF Viewer Pro subscription, and when running PDF Viewer on an iPad with iOS 11 and above, you'll be able to export pages out of the Document Editor to other applications via Drag & Drop.
To do so, head over to the Document Editor, (open your Document → Thumbnails → Edit) and you'll be able to drag pages out of the document to other applications on your iPad.
Please note that:
If you drag just one page, it will be exported as a single page PDF if the receiving application can handle PDF drops (e.g. Files.app).
If you drag just one page, it will be exported as a JPEG if the receiving application can handle image drops (e.g. Photos.app).
If you drag multiple pages out of PDF Viewer, they're going to be exported as separate JPEG files (one for each page) if the receiving application can handle image drops (e.g. Photos.app).
Currently there's no way to use Drag & Drop to export multiple pages into a single PDF file.