Saving histograms into a pdf / png format directly from ROOT

ROOT
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You could also save directly the canvases as PDF at the same time that you write the ROOT file, rather than doing that in a separate script later on.

To save all open canvases in a ROOT session, just type the following in a terminal:

        TIter next(gROOT->GetListOfCanvases());
        TCanvas* c = nullptr;
        while((c = (TCanvas*)next()))
        {
            const TString canvName = c->GetName();
            c->SaveAs(canvName+".pdf");
        }

You could do something similar if the canvases are not open, but rather inside a TFile.
Just iterate over all keys in the TFile and only call SaveAs if the key->ClassName()=="TCanvas".