Pedernales - Anse a Pitre crossing is official
The crossing between Pedernales and Anse-a-Pitre is an official and recognized border crossing, as long as you pass through the respective immigration/emigration offices during official opening hours.
Ofcourse you can cross the border like most locals outside the official hours if you merely intend to conduct local trade or be inside Haiti for the sake of saying that you have been in Haiti before returning to the DR. However, if you intend to travel further inland into Haiti, I strongly recommend that you enter through the official channels - it can otherwise cost you dearly, both in terms of money and refusal of renewed entry into Haiti. Despite what many may think, then the Haitians can be pretty contientious about who enter their country, and make the required checks when and if required - and it really is little hazzle to make it official.
When leaving the DR, you either visit the Pedernales emigration authorities at their office at the southern part of town and get your exit stamp before taking a motoconcho the 1 km to the border. On the Haitian side, you visit the container hosting both customs and immigration, right on the other side of the pedestrian bridge across the (usually dry) river, and get your entry stamp. The border is open -if I recall- from 7 am. to 5 pm.
From Anse-a-Pitre, you take a boat plying at night between Anse-a-Pitre and Marigot, (a bit of an experience admittendly considering the amount of cargo and passengers loaded on top), and from there a local but or 'tap-tap' to Jacmel or Port au Prince, or the occational truck loaded with charcoal and used clothes to Thiotte and onwards to Font Parisien and PAP.
Good luck