how to transfer pdf files from computer to iphone 5s using iTunes?

You need to have an PDF reader on your phone to transfer the PDF's. iBooks is a free app that reads PDF's and there are others.

Generally, get and download and install the app. Then, with your phone connected to your computer, iTunes open, go to the apps tab in iTunes and scroll down. You should see the apps that have data transfer ability.

You can then import the PDF's into iTunes and transfer them

I believe in the case of iBooks, there's a books tab in iTunes. You'd open that, click file, import, import to library. navigate to and choose your PDF's. iTunes will import them then you can choose which ones to sync.

Once you have the app, you can also e-mail the file to yourself. Open that file on your phone. There'll be a 'send to' dialogue where you can send the PDF to the app and use it to open the file.