1. Is document management part of free plan ?

Currently our free plan does not include support for document management. To get a feel of document management capabilities in Zoho Projects you can take a quick look at the video tour of Document Management

2. Can I have more disk space for an existing plan if it exceeds the allocated limit ?

No. Basically if your disk space exceeds the allocated limit for an existing plan then you need to upgrade to a higher plan to get more disk space.
For more details see Plans and Pricing

3. What file types are supported in Zoho Projects ?

You can upload any file types like .doc, .xls, .xlsx, .docx .pdf, .pps, .ppt, .avi, .jpeg, .wmv etc

4. How can I import my files from Google Docs into Zoho Projects ?

To upload your documents from Google Docs into Zoho Projects :

1. Go to Documents Tab and click on Upload New File
2.In this select "Browse using Google Docs"
3. Provide your Google login credentials to access documents from Google Docs
4. Now select the required files and upload them in Zoho Projects.

5. What will happen If I tried uploading a different filename using Upload latest version option ?

If you upload a file with a different name or file type a warning message "Filename mismatch" will get displayed for you.

6. Do you include support for sub-folders in Zoho Projects ?

Currently we do not support sub-folders for documents and we might include this option in one of our future updates.