In short: if Axcrypt can’t open the file after decryption then it can’t send through “Open in” to another app.
Good: Axcrypt can decrypt a .pdf, can open it, so can send it to “Adobe Acrobat Reader” app.
Bad: Axcrypt can decrypt .odf, can’t open it, so can’t send it to “OOReader” app.
If you can open a PDF file using AxCrypt then the problem is with OOReader.
When you use ‘Open In’ feature you’ll be given a number of options, e.g. what program to use. There are two rows in iOS. If you scroll to the end of each you see a “… More” button. Click it, find OOReader, toggle the switch to green and then press ‘Done’. Remember to repeat this for the second row of icons.
Svante correctly said:
This does require the app itself to register that it can open the file in question
If OOReader doesn’t register itself with iOS then AxCrypt can’t use it.
To test where the problem is, email yourself with an ODT file. When you receive the email, try to open the attachment. If you can’t then that means OOReader isn’t registered in iOS. If you can open the file (using ‘Open In’), please post back on here.