You need to have the paid version of Acrobat to save your PDF as a JPEG. The free Reader can’t do that.
Adobe Acrobat doesn't exist for iOS.
Sorry for the delay in response.
In the Adobe Acrobat Reader mobile application, you can only convert PDF to Word, Excel, PowerPoint or RTF files if you have the subscription.
To convert PDF to JPG on iOS device, you can do it online here Adobe Document Cloud on your mobile device.
Open the link and sign in with your Adobe ID and password.
Hope it helps.
Let us know if you have any questions.
I've opened the supplied link and logged n, but can't seem to find the file I want to convert (which is downloaded in safari so exist on the iPhone downloaded docs list, also opened in the iOS acrobat app, but still doesn't show up on the recently opened files list).