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Delaware LLC & federal tax.

Sole membership LLCs will be taxed through (Like S-Corps). So your regular income tax in Bulgaria will apply.
That's only if Bulgaria recognises the LLC as a pass-through entity (or doesn't bother to look into it). UK and Canada for example treat LLCs as taxable companies. It's probably untested in most jurisdictions, and interpretation may vary depending on how the LLC is effectively controlled and managed.

Odds are this is uncharted territory, where the US aspect is clear (no tax would apply under normal circumstances) but what happens in Bulgaria is uncertain.
 
That's only if Bulgaria recognises the LLC as a pass-through entity (or doesn't bother to look into it). UK and Canada for example treat LLCs as taxable companies. It's probably untested in most jurisdictions, and interpretation may vary depending on how the LLC is effectively controlled and managed.

Odds are this is uncharted territory, where the US aspect is clear (no tax would apply under normal circumstances) but what happens in Bulgaria is uncertain.
In Bulgaria the dividends from foreign companies are taxed with 5%.
But my question was if I must pay a federal tax in any case in U.S.
 
In Bulgaria the dividends from foreign companies are taxed with 5%.
But my question was if I must pay a federal tax in any case in U.S.
As a general rule, the answer is no.
If you live outside the US and operate your business alone, or you have employees who help you but all of them are located outside the US, then you generally should be able to operate your business without paying US tax.