There is no PDF Reader in the "Open in" dialogue

You can save a copy of the file to the PDF Reader app from within other apps using the Open-in dialogue (Only files and web pages can be exported to PDF Reader, but the web pages will be saved as PDF format). 

If PDF Reader is not in the list of available apps, please make sure the latest iOS version is in use on your device. Then 

1. Scroll the list of apps until you see the "More" button
2. Tap the More button. Then add and drag the PDF Reader app icon to the top of the list
3. Remove the apps you do not need in the Open in dialogue
4. The PDF Reader icon will be available in the Open-in extension list

You can also save files to a specific folder in the PDF Reader app using the Apple's Files app  (using saving the attachment from Mail app as an example):

1. Open the file you’re currently working with (e.g. Mail app) and tap an Actions button.
2. Tap “Save to Files”.
3. When the Files app is launched, tap the “On My iPad” (or On My iPhone)
4. Find out the PDF Reader folder and select a destination you would like to save the file to. 
5. Then the file will be saved to PDF Reader