You Do Need power plug adapter while traveling from Palau to Bulgaria.

Here is Why You Need An Adapter

The outlet and plug type used in Bulgaria are C and F and in Palau are A and B. This is how they look like.

Palau

In Palau, outlets of type A and B are in use.

Type A

Type B

Bulgaria

In Bulgaria, outlets of type C and F are in use.

Type C

Type F