Description on reports in NAV 2013

The description field for financial item (eg relating to a posted invoice) only shows the text: “Invoice xxxxxx”. How can I get the description from the invoice line shown in financials also?

I suppose you mean Description field in GL - it is so by design.

When posting documents to GL, their lines are grouped together by GL Account and Dimensions (in later versions Dimension Sets), so there is no way to carry over Descriptions from individual document lines.
If your, say, Invoice, has 20 lines with the same Posting Groups (that determines GL Account) and same Dimensions (Sets), in GL there will be only ONE summarized entry in GL with no place to store all 20 different descriptions.

However, there is a field Posting description in document header (by default not visible) - with the help of your implementing Partner company you can make use of it, but again - this description will be common to all combined lines in GL, if there is more than one.

I should add that trying to create reports based on Documents tables rather than Ledger tables (as this seems to solve the Descr issue) is a bad idea in most cases. The only exception is reprinting a posted document, never try to report financial info from Document tables, especially costing data, as those are NEVER updated after initial entry, which was “last known at the time of entry” - thus approximate and INACCURATE in most cases.