you have to use loadfromfile like below example , I used parts under Site folder,but you can load parts into products , adding below
-CONT_PATH "wt.inf.container.OrgContainer=<Org_name>/wt.pdmlink.PDMLinkProduct=<Product_name>"
<?xml version="1.0"?>
<!DOCTYPE NmLoader SYSTEM "standardX20.dtd">
<NmLoader>
<csvBeginCreateOrUpdateWTPart handler="wt.part.LoadPart.beginCreateOrUpdateWTPart">
<csvpartName>123</csvpartName>
<csvpartNumber>68</csvpartNumber>
<csvtype>separable</csvtype>
<csvgenericType/>
<csvcollapsible/>
<csvlogicbasePath/>
<csvsource>make</csvsource>
<csvfolder>/Default</csvfolder>
<csvlifecycle/>
<csvview/>
<csvvariation1/>
<csvvariation2/>
<csvteamTemplate/>
<csvlifecyclestate>INWORK</csvlifecyclestate>
<csvtypedef>wt.part.WTPart</csvtypedef>
<csvclassToUpdate/>
<csvorganizationName/>
<csvorganizationID/>
<csvsecurityLabels/>
<csvserviceable/>
<csvservicekit/>
</csvBeginCreateOrUpdateWTPart>
<csvContentFile handler="wt.load.LoadContent.createContentFile" >
<csvuser></csvuser>
<csvpath>test.xml</csvpath>
</csvContentFile>
<csvEndCreateOrUpdateWTPart handler="wt.part.LoadPart.endCreateOrUpdateWTPart">
<csvparentContainerPath/>
</csvEndCreateOrUpdateWTPart>
</NmLoader>
the PDFs should be under <WC-home>/loadfiles/content