What is the best project documentation software?

The question posed is unclear.
There are two elements to software documentation, one is handover documentation and other is related to the design and assurance process.
The handover documentation are as follows:1.
Software Revision Note2.
Maintenance/Installation Manual3.
Operating Instruction4.
Development Manual In relation to software assurance, these varies and is dependent on the SIL rating,you can refer to BS50128 for more detail.
However, as a minimum you would expec the followingt:1.
Requirements Specification2.
Concept Design/High Level Design3.
Detail Design4.
Unit Test Specification5.
FAT Specification6.
Interface Test Specification7.
VVM8.
V&V9.
Interface Control Document

Project documentation is used for controlling the way in which a project documents will be managed and the archiving the records on finalization.
The introduction of a software for this depends upon the project requirement and how we wish to manage it with a software.
If you are looking for a ready made software...
in an online environment...
I have worked with Aconex.
I have also worked in a network server based software Primavera Expedition..
in different projects.

Aconex is best software for project documentation

There are many softwares for project documentation among of them ERP Oracle System, Acconix & SAF. But the best software for Project documentation in Construction field is Acconix. 

In my experience A-site, PCM (Primavera) are the best systems to use for controlling projects. Easy to use for existing users and new users. SAP is also a good tool, but it can be a complex system. 

Microsoft Excel, Word, Presentation. And Auto cad

PCM(primavera contract management) is the best software.