Drop a DWG here. It opens. CAD viewer.

DWG, DXF and DGN. 10 MB per drawing. Deleted after viewing. The CAD converter exports DWG and DXF to PDF, up to 5 MB.

Three formats, three different origins

DWG — Autodesk's native drawing format. Binary and closed.
DXF — the published text interchange version of the same drawing. Larger, and readable by far more software.
DGN — MicroStation, from Bentley. Different lineage, same job.

Half of a drawing set is not in the drawing

Attached xrefs, SHX text fonts and .ctb plot styles are separate files on the sender's machine. A drawing that arrives sparse is usually complete and missing its attachments. Ask the sender for the whole folder, or for a bound copy: XREF, then Bind.

Over 10 MB

In this order:

PURGE — drops unused blocks, layers and styles. Often halves the file.
AUDIT with fix errors on.
-WBLOCK — writes out only the objects you need as a new drawing.
Save down to an older DWG version.

Do not convert to DXF to shrink it. DXF of the same drawing is bigger.

Blank, or nearly

Check three things before assuming the file is damaged.

Layers switched off or frozen. Common in issued drawings, and indistinguishable from an empty file until you turn them on.
You are looking at model space and the sheet is a paper space layout, or the reverse.
Custom objects from a vertical product render only if the drawing was saved with proxy graphics.

Password-protected DWG

AutoCAD removed drawing passwords in the 2016 release. If yours has one, open it in an older AutoCAD, remove it under Save As, Tools, Security Options, then upload.

Drawings to send elsewhere? The CAD converter turns DWG and DXF into PDF.