Is it possible to add custom metadata to file

If using NTFS you can store whatever you like in an Alternate data stream

As per Jesper's comment, you can use the DSOFile library to read and write to Custom properties that are stored in ADS.
Works well for me though note the fact that the properties are lost when file is transferred to a different file system, including email. see http://www.keysolutions.com/blogs/kenyee.nsf/d6plinks/KKYE-79KRU6 for a 64bit implementation, link to MS original and comments.

I know that each file has metadata like title, subject, keywords and comments

That is not true. Most file types do not have a 'standard' form of metadata.

In particular, PDF files don't have properties that Windows Explorer recognizes.

Metadata (file attributes) is not a function of the filesystem.

• Office files use a structured format that allows for such attributes.
• Jpeg ues EXIF, a different format