Architects, engineers and contractors send DWG, DGN and DXF files to people who do not own AutoCAD. The CAD viewer opens them in the browser: no install, no licence, no account.

Viewing DWG, DGN and DXF drawings in the browser

The three formats

DWG — AutoCAD's native format, and the one most drawings arrive in.
DGN — MicroStation's format, common on infrastructure projects.
DXF — the interchange format, written by almost every CAD application.

All three are accepted, up to 10 MB per file. Over that, the upload is refused and the message names the limit.

What the viewer does with a drawing

Renders from the drawing data. Layers, line weights, hatches and dimension text come from the geometry, not from the preview thumbnail stored inside the file. Title blocks stay legible at full zoom.
Layer control. Switch layers off one at a time — hide the hatching or the services layer to read the walls underneath.
Zoom and pan. Move around a dense plan at full precision.
Server-side rendering. The drawing is rendered on the server and drawn in your tab, so a phone or an old laptop handles a complex plan without a graphics card.
Nothing installed, nothing kept. No AutoCAD licence, and the upload is deleted after viewing.

The engine underneath is the Doconut .NET library, which is also what a development team licenses to put the same CAD rendering inside their own application.

Where the limits are

Say them out loud, because a drawing set will hit them:

10 MB per file. A large multi-sheet DWG will exceed it; send the single sheet that matters, or purge unused blocks and xrefs before exporting.
One file at a time. No folder upload, no ZIP, no batch.
No sharing. There is no link to send a client and no account for them to log into — each person uploads the copy they were sent.
2D reading, not editing. Nothing here modifies the drawing.

Who it helps

Clients and stakeholders who need to look at a drawing without buying CAD software.
Site and office staff on machines where nothing can be installed.
Anyone triaging a file — open it, confirm what it is, then decide who needs the full CAD seat.

Conclusion

For reading a DWG, DGN or DXF under 10 MB, a browser tab is enough: OnlineDocumentViewer.com. For CAD rendering inside your own product, the library is at Doconut.com.