Thank you both for your help.
Greenrig - I have a laptop, my husband has a tablet and the same thing happened to him too, which is why I thought it must have been a Windows 11 update. However, when I googled the problem I saw some people had this problem (photos opening in pdf) a few years ago before Windows 11. My laptop is a Lenovo as is my husband's tablet. "They" in the sentence about them changing from jpg to pdf are the photos. I've done what you suggested and unticked "hide extensions for known file types". The photos do show jpg now.
Gan Yam - now I've changed the settings for photos to jpg they open, but as you say. the photos open small on black screen, but that's not how they used to open either. Previously, when I clicked on a saved photo to open it, it just opened on the screen without anything around it. I don't like the way it opens now and I don't see the point. I could always enlarge the photos if I wanted to and I prefer to open them a normal size, not a small size and then have to enlarge them. You say
I also found that Windows 11 has a default setting that does not allow you to download any programmes/apps which are not approved by Microsoft. I had to turn this setting off (no going back after its turned off and something that Microsoft does not wants you to do) but this then allowed to download the programmes I wanted to use my laptop for.
How do you do that?
I have Windows 11 because something happened to my laptop last year and everything was wiped from the hard drive. The computer engineer said I must have done something but all I did was close down the laptop before I went to bed and in the morning nothing would open. Sadly, the exterior hard drive wasn't programmed properly (a previous computer engineer thought OneDrive was enough - obviously not in my case), so I only had everything backed up to 2021. Fortunately, I've managed to retrieve all important programs/files and folders from copies I'd sent to other people. Anyway the computer engineer installed Windows 11 and everything is still saved to OneDrive. I have made sure the exterior hard drive is set to back up weekly too. I know I would have had to move to Windows 11 eventually but I'm not particularly happy with it. As someone said all the upgrades etc. seem to be just to suit programmers not ordinary people who use the computers.