I want to buy this transmitter/receiver but in specifications I see everything about audio only. Can I use it with my PC to connect keyboard or mouse?
Based on the information and drawings on the link you've provided, it appears that the USB port is supplying power and very likely power only. The double-ended cables provide for audio source/destination for the bluetooth aspect of the device.
This means the answer to your question is quite likely NO.
A quick search discovers a product available that explicitly presents as mouse/keyboard capable: