Moldovan citizens can use simple passports to go to Ukraine. There are no restrictions to use this document to pass the border between Ukraine and Moldova.
From the very beginning of this year, Kiev introduced the passport control at the border, but, the restrictions are not valid for Moldova. Between the authorities of those countries there is an agreement that permits Moldovan citizens to be on Ukraine's territory without visa.
The decree according to the passports checking at the border was signed by Petro Porosenko, the President of Ukraine, in 2017.
It is obligatory for the Russian citizens to present their passport while entering on the territory of this country.