I found one called TurboViewer.
I need something like that for situations where my client and/or others I am coordinating with have a need to take a look at .dwg files in the project dropbox and all we have available at the time is our iPhones. I don't always have time to make BIMx files, and some of the .dwg files in the project dropbox will stay as .dwg permanently.
I know of a good solution but is costs $10.00/mo A360. https://a360.autodesk.com/
I recently demoed AutoCAD 2018 and installed A360 on my iPad and iPhone. The AutoCAD 2018 demo has expired (very nice by the way) but apparently A360 has not expired yet.