May your existing app seems 32-bit applications, thats why some of it features does not work well with new iOS version. Now Apple turns off support for 32-bit applications.
To avoid this type of barrier I will use Objective C v2.0 to migrate your code with latest iOS version which will be released in future.
It will be great if you can provide us a build/.ipa of a existing iOS app/Android app. So we can mapping that functionality into new development. We have all type of mobile devices to supports any older app.
There are not any error descriptions? which you've shared are only some ViewControllers; what is being assume of there is - your app "My PDF Doc" written by Objective C; Have some SDKs/Libraries i.e Dropbox SDK, Google AdMob, Goole Analytics etc. Besides, I have been searched into iTunesConnect, your app is not in there: Hence, it will be the best if I can get the source code. I can find out which API is revoked from latest iOS version, accordingly to this I can migrate this API with the latest version of iOS.
➥ About Regular Updates:
My availability is 24 hours. You can pine me & even call me on emergency as well
➥ Free Bug Fixing Support Model:
We're offering only 2-3 months to provide support and maintenance with fix bugs
➥ Deadline & Hiring Steps:
For now I can not finalise about the total estimated time of arrival. To set out the deadline, the above doubts will have to be make a settlement.