Some of these I've scanned myself, most of the others were scanned by people I know.

A Soviet history of Europe and the Americas covering 1640-1870: https://archive.org/details/ModernHistory16401870

A Soviet history of the world covering 1917-1945: https://archive.org/details/AContemp...istory19171945

USSR: https://archive.org/details/HistoryUSSREraSocialism (covers Soviet history from 1917-1980)

China:
* https://archive.org/details/in.ernet.dli.2015.98145 (covers 1919-1956)
* https://archive.org/details/YearsOfTrialTurmoilTriumph (covers 1949-1980s)

United States: http://uselectionatlas.org/FORUM/ind...topic=202462.0 (all sorts of Marxist works on US history here)

Poland:
* https://archive.org/details/TopolskiPoland1986 (entire history of Poland up to the 1970s)
* https://archive.org/details/ClassStr...ocialistPoland (covers Poland from the early 20th century to the early 80s, with a focus on the workers' movement followed by an analysis of the Polish state and economy after 1945 and rise of Solidarity)

Germany:
* https://archive.org/details/TheLesso...nyGuideHistory (history up to the end of WWII)
* https://archive.org/details/GDRHistoricalOutline (history of the GDR going up to 1979)

Macedonia: https://archive.org/details/HistoryMacedonianPeople

Bulgaria: https://archive.org/details/EvansShortHistoryBulgaria

Mongolia: https://archive.org/details/HistoryOfTheMPR (entire history of Mongolia up to 1970 or so)

Central Asia: https://archive.org/details/CentralAsiaInModernTimes (from the early 19th century onward)

Korea: https://archive.org/details/ModernHistoryOfKorea (covers 1919-1970s)

Vietnam: https://archive.org/details/OutlineH...amWorkersParty (covers 1930-1975)

Cambodia:
* https://archive.org/details/ChinaCam...ietnamTriangle (focused on the rise and regime of Pol Pot)
* https://archive.org/details/Kampuche...olutionRescued (ditto except also has some stuff on Cambodia in the 80s)

Albania:
* https://archive.org/details/TheHisto...iaABriefSurvey (whole history up to the 1950s)
* https://archive.org/details/TheHisto...ctionOfAlbania (covers 1944-1975)

Afghanistan: https://archive.org/details/AHistoryOfAfghanistan

African countries (as well as Africa itself): https://www.revleft.space/vb/threads...-Africa-(PDFs)