PreCAD is a 2D CAD application.
You can use many drawing tools without any restrictions.

PreCAD saves files in its own format called "pcad" format but it supports reading DXF JWC JWW (jw_cad) files and writing DXF JWW files (but there are any unsupported elements functions and so on).
Although It is experimental you can read PDF HPGL SVG and save PDF PNG HPGL SVG.

You can use the "jws" format of jw_cad as the parts.

In addition to layers there is a function called "sheet". You can set the scale of "Sheet". By using multiple "sheets" you can mix multiple scales in one drawing.
Also you can create multiple pages.

manual(sorry Japanese only)
https://junkbulk.com/android/PreCAD/manuals/PreCAD_android_manual_2_jp.html