PDF updater

What do you mean by update, like edit the content? The PDF file format wasn't specified with editing in mind, so you have fragments of text individually placed on the page, for example. Although possible, it's not the best way to edit a PDF file and there aren't a lot of tools doing a good job. If you have any way to get the original editable format (e.g. latex source or some office document like .odt, .docx and the like), that's definitely preferred.
If someone sends a form as a PDF which needs to be filled in, how do you go about filling it in?

I'm sure Adobe provide some program for Mac and Windows which will do this.
Ok, different story, the fields in a PDF form are of course designed to be edited. So at least we could clarify the problem in question here ... sorry, can't help you further for my lack of experience with PDF forms, but I guess someone else can :)
Installed it yesterday but it didn't run.

Today I tried it from a terminal window:-

root@ThinkPad:~ # evince
/usr/local/lib/libgtk-3.so.0: Undefined symbol "g_param_spec_get_name_quark"

What should I do?
configure: error: Package requirements (gtk+-3.0 >= 3.14.0 gio-2.0 >= 2.36.0 gmodule-no-export-2.0 >= 2.36.0 gmodule-2.0) were not met:

Package dependency requirement 'gio-2.0 >= 2.45.8' could not be satisfied.
Package 'gio-2.0' has version '2.44.1', required version is '>= 2.45.8'

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

I think I'l give it a miss and find some other solution. evince seems like too much hard work.
I am able to fill out my US Tax forms with graphics/evince and save the completed form.
I prefer a different approach: I convert those tax pdfs into png flies (using graphics/gimp or graphics/ImageMagick/) and set them as backgrounds in editors/libreoffice spreadsheets. Then I adjust the grid to fit the form's fields. Thus a lot of values can be simply calculated and/or referenced from other sheets. Of course, you have to spend some time once, but then it's very easy to adjust those things next year with minor changes.
This tax cycle, with a new printer that emulates postscript, I filled out the forms in evince and converted the pdf to ps using pdftops. Using my printers duplex queue (lpr) I was able to print the completed forms I mailed to IRS. aragats method is intriguing - it would be a key step in a TurboTax style package. The spreadsheet could do some of the calculations, tax tables could be parsed etc.