Google Play Books Question

[fatyank]

I'm using Google Books as my reader. I've uploaded several pdf files and created books (i.e. study manuals, company brochures etc.). I want to be able to highlight them as I read through but I am unable to. I went to help section and it says to press and hold area you want highlighted then choose which color you want to use. When I press and hold, nothing happens. I tried on several different files I uploaded from pdf. I then tried this procedure on books I purchased and it worked fine.
Is there anyone using Google Books that knows how to use highlight on uploaded pdf files. I don't really want to use a different reader.
Thanks

The Fatyank