Usually I receive a PDF contract that I need to sign and send back online. I am wondering what is the easiest way to do so on computer, to avoid the hassle of print/sign/scan process?
Xournal is a good interactive option to write on top of existing PDF. It supports digital pen or you can paste/import your signature.
sudo apt-get install xournal
There are best tools available in Ubuntu for pdf editing like PDF-Shuffler, pdftk, inkscape. Visit this link for pdf editing in different scenarios.
For your purpose an easy way I do it is using GIMP. You can import a pdf file (or one of its pages) into GIMP. It has native pdf support. You can then add your signature (either draw it or copy it from another file). This works nice for single page pdfs.
For multiple page pdfs you can check how gimp handles them here.
You could try installing Oracle PDF Import Extension in Libreoffice. You can find it here.
Quoted from http://code-industry.net/free-pdf-editor.php
Master PDF Editor provides you advanced functions for working with PDF and XPS formats:
Create new PDF and XPS files or edit existing ones.
Add and/or edit bookmarks in PDF files.
Fast and simple PDF forms fill out
Changing font attributes (size, family, color etc)
Encrypt and/or protect PDF files using 128 bit encryption.
Convert XPS files into PDF.
Add PDF controls (like buttons, checkboxes, lists, etc.) into your PDFs.
Import/export PDF pages into common graphical formats including BMP, JPG, PNG, and TIFF.
Signing PDF documents with digital signature, signatures creation and validation.
Free PDF Editor on Linux ( for non-commercial use)
Installation of Master PDF Editor
Now use the dpkg command to install Master PDF editor Debian package.
sudo dpkg -i master-pdf-editor_*.deb