If you have issues with particular files in either PDF Viewer for iOS or Android, it often helps a lot to identify the root cause of these issues when you share the files with us.
The best way to do this is to upload these files to a cloud service that has the ability to share files, like Dropbox, Google Drive or iCloud Drive. When you upload these files there, you can then create a share link and send this link when talking to our support.
You can also attach files directly to the email when contacting us, if you are sure the file is small (not larger than a few megabytes).
If the file you are trying to send us is large and you don't have access to a cloud provider, please ask us in support. We can generate you a link to a cloud provider where you can then upload a single file.