The options, honestly ranked
Autodesk's free DWG TrueView is a genuine full-fidelity viewer and it is Windows-only. If you have a Windows machine available, it is a reasonable answer and costs nothing.
Free browser converters work anywhere and cost you the file. The drawing is uploaded, usually flattened to a raster or PDF, and layer and block structure is lost. For anything covered by an NDA this is not an option at all.
A full CAD subscription solves everything and costs hundreds per year, which is absurd for reading a drawing twice a month.
A local viewer on the device you already carry covers occasional review: no upload, structure preserved, one payment.
What you give up without AutoCAD
No editing, no dimension authoring, no plotting to a titleblock, no xref resolution across a project tree, and no certified output. A viewer answers “what is on this drawing”, not “change this drawing”.
Approximate on-screen inspection is fine for understanding a layout. Anything that feeds fabrication, safety or compliance has to be verified against the drawing's own annotations and the source CAD system.
Reading a drawing on a phone
A DWG on an iPhone is genuinely usable if the app parses locally: pan and zoom the model space, isolate the layer you care about, and stop carrying a printed sheet that is two revisions old.
Because the parsing is local, the drawing opens in a plant room or a basement where a converter-backed app would show a spinner.