DTP stands for Desktop Publishing. It involves using software to format documents for printing or for online distribution.
Localization DTP involves the adaptation of the translated documents so that they are correctly formatted in their translated versions. This can be a simple process or it can be a complicated process. It depends on how the original is designed, what software was used and internationalized the original is e.g. does it have graphics with uneditable text?
For source documents in PDF format when the original editable document is unavailable, a DTP step may be needed to recreate the editable original document. When translating a document into multiple languages this will save time and cost after translation.