DWG opens only in CAD software, and a file saved by a newer AutoCAD can fail to open in an older one. PDF looks the same on every device and keeps several pages in one file. DWG is at home in architectural plans, mechanical parts and engineering layouts that are still being edited; PDF is the usual choice for documents that must look the same everywhere, and printing.
The defaults suit most files. Under Options you can choose the page size.
The drawing is shown as its model space, fitted to the extents of what is in it. If model space is empty, the first layout that has objects is drawn instead. Objects on layers that are switched off or frozen are not drawn. It reads AutoCAD DWG files from R13 (1994) to the 2018 format that AutoCAD 2018 to 2025 use; older and newer files are refused with a message. The PDF is a vector page, A4 by default or A4 or US Letter as chosen under Options, turned to suit the drawing. Text set in a TrueType font is real, selectable text; text set in an AutoCAD SHX font is drawn as lines, so it looks right but cannot be selected or searched. The browser has none of the fonts a drawing names, so text is drawn in a built-in font with Latin, Greek and basic Cyrillic letters and the usual symbols (m², Ø, ±, °). Chinese, Japanese and Korean text appears as empty boxes.