I have a simple wordpress site.
There are few sites such as www.medstudy.com. They have free access to PDF file on one subject ( pulmonary Medicine) to be reviewed freely. Any body can register freely and have an access. Those who like this can get access to other subjects. They need password which is give by registration and payment. Try this link to get better idea.
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People can not download or save this on your computer. I know computer geek can do any thing.
My question is that. I have a PDF file. People can view that on our site by opening but not able to download / save or copy. Every time they need to go on our site. They need log in and password for access. This will be given for a limited period ( 6 month or year) and then it expires.