I have Hp 15-d107tx notebook pc. It have 4GB of ram and I want to upgrade it to 8GB but i don't know how much my laptop support. I checked inside my laptop, It have 2 slots for RAM and elpida 4gb 1rx8 pc3l-12800s-11-12-b4 This RAM is installed in my laptop.
You can use another 4GB with same. Only Make sure its 4GB DDR3L (L indicates Low voltage RAM).
Try to purchase same speed of 1280MHZ. Max Speed it support is 1600MHZ.
If you have two slots, and one slot has a 4GB RAM module in it, then the other will accept 4GB as well. However, you want to know if it can accept 8GB per module or even more. The HP specs page doesn't give much info, doesn't tell the maximum allowed amount of RAM. The following page says it can work with 16GB total, so two modules of 8GB RAM.
This one says 16GB as well:
This means that it will probably work with one 4GB and one 8GB module, so 12GB total. Having two identical modules will probably speed things up, so if 8GB is enough, that is cheaper and faster.