Citing a document without year information
First, if this is a student paper, ask your supervisor; if it's a journal article or similar publication, consult the style-sheet. 

I've seen 'n.d.' for 'no date'. For online sources, it's a good idea to include the last access date, like so: 


  Brown, Alice (n.d.), On the Flubbication of Foobars, Report, Institute for the Study of Things, Boulder, Co., http://thingsistitute/pub/flubbication.pdf (accessed 6 June 2018).