after combing files I cant edit them on Ipad

I have converted some documents from Word on adobe acrobat DC. This has been simple. I have been using my mac book pro to do this. When I then open the pdf documents using the acrobat app on the iPad they open and I can edit them as expected. When I combine files and then try to open in acrobat on an iPad it comes up with the message "MODIFYING THE DOCUMENT IS NOT ALLOWED BECAUSE IT IS AN ATTACHMENT" please help with this it makes no sense why combining files restricts me from editing the forms after. Just to also mention I have checked the document security settings and everything is allowed. 

Help!