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This release has a lot of changes to adapt it to new features of Drupal core.
Some settings has been moved from module settings to field instance settings, so, you can have several fields showing previews of pdf files on differents manners, and this gives the formatter much more power if you use it on views.
Now you can select which image style to use, instead of fixed size from 7.x-1.x release.
Some code has been suppressed in favour of image module, and some in favour of file module, in order to keep this code as small as possible, and potentially reducing bugs.