Insert Salesforce file and link it to sobject

CreateSalesforceFile

PageReference pg = Page.EHSC_OSB_PDF;
pg.getParameters().put('id', oppId);
ContentVersion conVer = new ContentVersion();
conVer.ContentLocation = 'S'; // S specify this document is in SF, use E for external files
conVer.PathOnClient = 'OSB '+DateTime.now().getTime()+'.pdf'; // The files name, extension is very important here which will help the file in preview.
conVer.Title = 'OSB '+DateTime.now().getTime()+'.pdf'; // Display name of the files
if(!Test.isRunningTest()){
    conVer.VersionData = pg.getContentAsPDF(); 
}
else{
    conVer.VersionData = Blob.valueOf('Appointment Form'); 
}

insert conVer;

Id conDoc = [SELECT ContentDocumentId FROM ContentVersion WHERE Id =:conVer.Id].ContentDocumentId;

//Create ContentDocumentLink
ContentDocumentLink cDe = new ContentDocumentLink();
cDe.ContentDocumentId = conDoc;
cDe.LinkedEntityId = oppId; // you can use objectId,GroupId etc
cDe.ShareType = 'I'; // Inferred permission, checkout description of ContentDocumentLink object for more details
cDe.Visibility = 'AllUsers';
insert cDe;