Let's say I have to fill out this form and email it:
http://www.ukba.homeoffice.gov.uk/si...isas/vaf4a.pdf

It doesn't have fillable form boxes.

Is there a way to write over the top of the forms as if it was an image?

Where can I learn about pdf on linux? Is there a mailing list that is particularly active? I get spammed with adverts when searching for resources for help on google.

Yes...you save it as an image, and use an image-editor to put in what you want. Inkscape can do it, and I *THINK* the Gimp can open a PDF as well. Save it as a JPG or some other image file, and put in whatever text you want, using the text tool. And be advised that if you're filling out a government form, they may WANT it in your handwriting, especially for something like a visa.
There is no 'PDF on Linux'...the PDF standard is the same across ANY platform. And not sure what you mean by spammed with adverts while searching on Google...I've not seen any. Unless you mean, the commercial pieces of software?