The way I am reading this, even if you went paid you'd barely cover your costs (given how time consuming it is for you to bug fix). Plus without the free "trial" you'd struggle to attract paid customers, but while the paid "trial" exists people can just re-register to get all their files or otherwise only needed one page anyway so won't pay.

So my question is this: How much does running the service cost? If it is fairly inexpensive to run you could go a mixed "ad supported" and freemium model, with zero more bug fixes. Just sell what you have.

Instead of selling PDF conversion you sell convenience. You give your paid users an identical product but allow them to queue up dozens, even hundreds, of files via some basic desktop program maybe. Alternatively auto-magical conversion via email (PDF comes in, plain text goes out). Plus turn off ads (e.g. make free users wait 20 sec on an ad page).

The vast majority of your revenue would come from adverts, and while it might never be a screaming success, it might at least make a little profit year upon year and eventually pay for the effort you put into it.

But if it is expensive to run as is then I have nothing. You'll just have to try to make the paid model work.