The grammar is different, but they are both correct. 'Jordanians' is a noun, while 'Jordanian' (as in 'Jordanian people') is an adjective. They mean the same thing, however. 'Jordanians' is the same as 'Jordanian people'. This refers to the entire population.
If you want to talk about only one section of the population, you'd use the adjective to qualify whatever noun you're referring to e.g. "Nearly all Jordanian men smoke."
Note that this flexibility is only usually possible with nationalities that end in 'an' or 'i'. For example, you can say 'Nearly all Korean people' or 'Nearly all Koreans', 'Nearly all Kuwaiti people' or 'Nearly all Kuwaitis'. With many other nationalities, it is not possible because there is no noun that you can make from the adjective with eg French, Dutch, Irish (other than the slightly outdated and gender-specific 'Englishman' and so on). With others, there is a noun - known as a 'demonym' - but it is sometimes also a little old-fashioned ( e.g. Spaniard).
If in doubt, use the adjective.