Off[General::spell]
Off[General::spell1]

Clear["Global`*"];

Replace  path with your specific  path:

<<"Dropbox/teach/PY751/loadCTEQ5.m"

Defined functions

?cteq5pdf

parton[0]
cteq5pdf[1,0,.1,10]

Examine pdf vs. x

iparton=1;
Plot[x cteq5pdf[1,iparton,x,10] //Evaluate
 ,{x,0.01,1}
 ,PlotLabel->parton[iparton]
 ]

Evaluate pdfs at example point x=0.1

Table[{parton[ipart],cteq5pdf[1,ipart,0.1,10]} ,{ipart,-6,6,1}]  //TableForm

number of up quarks in proton

ups[xi_]:=NIntegrate[cteq5pdf[1,1,x,10],{x,xi,1}];
LogLogPlot[ups[xi],{xi,10^(-6),1}]