Could you provide some more details on the version of our controls you are using? The standard numbering we use is YYYY.Q.MDD which is something like 2018.2.710 for the second release of 2018 that was built on the 10th of July. With this in mind, version 1.0 would not be the version of our controls.
I also advise that you try sending another file to the user in the same context (in the iframe) to see if this works as expected. If not, the issue would definitely be with the way the browser handles this. Perhaps a browser extension, a firewall or antivirus software may be blocking the download. So, you can try this in a different browser, on a different machine or in a different environment to see whetherthis is the case.
Since this issue is quite peculiar, I suggest you open a ticket where you can send us the actual repro, with the Telerik assemblies you are using, and with a video of what you see. Adding a FiddlerCap of the network traffic would also help.