If you mail the file to yourself, are you able to view the attachment on your iPad/iPhone in the Mail app? If not, OmniFocus won't be able to display it, either.
Having established that iOS can display the file, the next thing to check would be that you've successfully embedded the file in your database. On the Mac, use the Window->Attachment List command to bring up a window showing all of the attachments embedded in your database. If it isn't there, it won't be synced to any other client.
If you can read the file in iOS, and you've successfully embedded it in your database, the next thing to check is that you've completed a sync on both the Mac (to put the embedded file in the sync repository) and then on the iOS device (to retrieve it from the sync repository). An easy way to make sure that all the necessary syncing has happened is to put a marker action in your Inbox on the Mac, then sync everything. When the marker action shows up in your Inbox on the iOS device, you know that all transactions prior to adding that marker action have also been synced.