Getting Purchase Orders filtered via date from OrderContext

OrderSearchOptions searchOptions = new OrderSearchOptions();
searchOptions.StartingRecord = 0; //or whatever you want to specify for paging purposes
searchOptions.RecordsToRetrieve = 10000

To retrieve only Purchase Orders specify 'PurchaseOrder' as a search parameter - 

 searchOptions.Classes.Add("PurchaseOrder");

The next thing is to build parameters against the table - 

// initializes search parameter options to be used while querying against db.
OrderSearchParameters parameters = new OrderSearchParameters(); 

StringBuilder metaWhere = new StringBuilder();

// specify your conditions, my case it should be the orders with GersOrderNumber column!=null
metaWhere.Append("NOT META.GersOrderNumber IS NULL"); 

//the dates against the Created Column of OrderGroup_PurchaseOrder table
metaWhere.Append($" AND META.Created>='{startDate}' AND META.Created<='{endDate}'");

parameters.SqlMetaWhereClause = metaWhere.ToString();

// Get the inner query to have OrderGroupId from PurchaseOrder table.
parameters.SqlWhereClause = "OrderGroupId IN (Select ObjectId FROM OrderGroup_PurchaseOrder)";

The last thing is to call the API - 

PurchaseOrder[] purchaseOrderCollection = OrderContext.Current.Search<PurchaseOrder>(parameters, searchOptions);

The API would return all Purchase orders within specified conditions.