The limitations of the free version of Melody Assistant are:

no clef or time signature change, export limited to a few seconds, watermark on printed pages.

There is currently a discussion going on in abcusers about converting a score in pdf to ABC: PDF to abc Conversion

I mentioned upthread that I use David Zemsky's Sheet Music Scanner and Reader on my Android tablet to do that - it converts from pdf, the camera, or a screenshot. It's not perfect but it's cheap. But that thread mentions Audiveris - which I'd not heard of and is free.

All these tools convert the score to musicXML whence you can import it into EasyABC, Michael Eskin's webtool, and probably several other programs.
But, as Roger says on that thread, if you you can sight-read dots and mentally convert them into letters (as opposed to buttons or frets) and if you know ABC well, then you may be better off just transcribing the tune manually.