The word(s) "doubter" appears 2 time(s) in 2 verse(s) in Quran in Muhammad Pickthal translation.

(1) And verily Joseph brought you of old clear proofs, yet ye ceased not to be in doubt concerning what he brought you till, when he died, ye said: Allah will not send any messenger after him. Thus Allah deceiveth him who is a prodigal, a doubter.

(سورة غافر, Ghafir, Chapter #40, Verse #34)
(Arabic, Transliteration, Urdu (Ahmed Ali), Urdu (Jalandhry), Farsi, Yusuf Ali, Shakir, Picthal, Mohsin Khan, French, Spanish, Indonesian, Malay, German, Bosnian, Russian, Dutch, Italian, Portuguese, Tafsir - Jalal ad-Din, Tafsir - King Fahad Quran Complex)

(2) Hinderer of good, transgressor, doubter,

(سورة ق, Qaaf, Chapter #50, Verse #25)
(Arabic, Transliteration, Urdu (Ahmed Ali), Urdu (Jalandhry), Farsi, Yusuf Ali, Shakir, Picthal, Mohsin Khan, French, Spanish, Indonesian, Malay, German, Bosnian, Russian, Dutch, Italian, Portuguese, Tafsir - Jalal ad-Din, Tafsir - King Fahad Quran Complex)