The method 'Skip' is only supported for sorted input in LINQ to Entities

Seems like the error is exactly what it is says. "Skip is only allowed on Sorted inputs". Searching for this error, I've found this.

It should be fixed if you include an OrderBy before Skip:

source.orderBy(???).Skip(PageIndex * PageSize).Take(PageSize)); 

Which might be a problem since you are passing a generic object T. You might need to expand your class to receive another parameter to indicate the order by element.

that is worked (use first IOrderedQueryable):

http://msdn.microsoft.com/en-us/library/bb738702.aspx

 IOrderedQueryable<Product> products = context.Products
        .OrderBy(p => p.ListPrice);

IQueryable<Product> allButFirst3Products = products.Skip(3);

Console.WriteLine("All but first 3 products:");
foreach (Product product in allButFirst3Products)
{
    Console.WriteLine("Name: {0} \t ID: {1}",
        product.Name,
        product.ProductID);
}

An IQueryable does not have an order, so saying "ignore the next x elements" doesn't make any sense.

If you include an order by clause (or possibly an AsEnumerable() call - untested) then your data takes an order and Skip and Take are now sensible.