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

Clear["Global`*"];

Replace  path with your specific  path:
(Or, if you are in a hurry, just open up the file loadCTEQ5.m  and execute the initialization.)

<<"loadCTEQ5.m"

pdf[x__]=cteq5pdf[x]
simplepdf[iparton_,x_]:=cteq5pdf[1,iparton,x,10];

Defined functions

?cteq5pdf

parton[0]
simplepdf[0,.1]

Examine pdf vs. x

iparton=1;

Plot[x simplepdf[iparton,x] //Evaluate
 ,{x,0.01,1}
 ,PlotLabel->parton[iparton]
 ]

Compute Momentum Fraction

mom= Quiet[Table[
  NIntegrate[ x simplepdf[ipart,x]  
  ,{x,0,1}]
  ,{ipart,-6,6}]]

{Table[parton[i],{i,-6,6}],100* mom} //Transpose //TableForm

Check Total Momentum Fraction

Plus @@ mom

Evaluate pdfs at example point x=0.1

Table[{parton[i],simplepdf[i,0.1]} ,{i,-6,6,1}]  //TableForm