Can't view pdf attachments within mail apps

iPhone 6P / iPad Air 2

both running iOS 9.3.1

Previously, when I tapped on an attachment in the Mail app I could view it. Now, it only downloads an image of the first page (which means I don't even have the option of opening into a pdf reader. All other attachment types can be previewed as always (including images and spreadsheets.

This also occurs to some extent in other mail apps, but the behaviour differs - sometimes it is just an image, at other times the whole pdf downloads but opens with a placeholder saying "PDFReader File", with its size, on a grey background. At least I can then open into another app. The apps that behave this way are Outlook, Dispatch and Airmail.

Spark works as previously - I can view pdf files within Spark without opening into another reader.

I tried deleting Spark, in case it was the issue, but nothing changed.

All pdf readers are behaving normally (PDF Expert and Documents). I tried deleting Documents because it was a recent install, but it also made no difference.

This is occurring on both iOS devices I use, and my husband's iPhone 6 as well.

What I have noticed in Mail that when I tap to download the iBooks icon fleetingly appears and I'm wondering if it has something to do with that.

I've tried a few other things, looked in settings, rebooted etc etc, but to no avail. It only started a few weeks ago so am wondering if something was introduced in the last update.

Many thanks if you can offer any advice or next steps.

Deb