Why in a PMOS transistor VDS has to be negative?
The answer is as simple as it gets: if voltage between drain and source is 2V, voltage between source and drain is -2V. It's exactly the same thing, just in the opposite direction.

In NMOS, the conventional current flows from drain to source. Thus you need to have positive Vds.

In PMOS, the conventional current froms from source to drain. But you measure Vds as voltage between DRAIN and SOURCE. Since you need Source-Drain voltage positive, Drain-Source will be negative.

Exactly the same logic applies to Vgs. Basically, Vab means Va-Vb.