WildeIdea, Keep It Simple gave the solution. Keeping an eye on "Finder Web Content" in the Terminal solves the problem.
That "Finder Web Content" may have a purpose in life (which one?), but you can well browse the Net and work after force quitting it. Before I found the solution in another blog it was using more than 4GB of RAM, and putting everything else out of the picture (even with 8GB of RAM). Conversely, once that FWC is gone, other tasks immediately start to reduce their needs.
It may be connected to Word but it seems to affect all applications. When the machine starts to slow down and you have that infamous pop-up "Your system has run out of application memory" it is time to visit your Terminal to regain speed in a minute or two after eliminating that nuisance.
Zheechen you are right, there seems to be an issue with the preview, but "Show Preview Column" seems to be missing in Mavericks.
Elsewhere someone also suggested to download Memory Clean (free app), which is a neat little helper.
Good luck.