JPG - 72dpi on iPad / iPhone; 150dpi on iPad Pro

PNG - 150dpi on iPad / iPhone or 300dpi+ with transparency; 300dpi and 600dpi on iPad Pro

SVG - an open-standard vector format. Not all SVG formats are the same, however, so be prepared for inconsistencies between apps.

DXF - a widely-supported CAD format created by Autodesk.

PSD (Photoshop) - with Layers, requires an Adobe Creative Cloud account.

PDF - both High-Res Flattened (maintaining look+feel) and simplified Vector paths (including basic, variable-width strokes).

CPT (Concepts native), which gives you a link that allows you to transfer complete drawings between devices and to other people for further editing.