This article covers the process to correct the issue with eDrawings viewer 2017 and 2018 when it fails to open your STEP or IGES file with an error "unable to open file".
If your computer does not have Microsoft C++ 2008 (for eDrawings 2017) and C++2013 (for eDrawings 2018) then STEP (.stp), and IGES (.igs) files will not open.
For 2018, You can download and install Microsoft C++2013 here:Click Download and Choose VCredist_x64 and VCredist_X86
Note: If you require support for the free version of eDrawings Viewer, please click here. eDrawings Viewer Support