krop is a free and useful tool to crop the pages of PDF files . It is available for Linux system. It is released under GPLv3+ licence and written in Python and relies on PyQT, python-poppler-qt4 and pyPDF. The main feature of this Krop app is automatically split pages into subpages to fit the limited screen size of devices such as eReaders. Using this app you can rotate your pdf and you can include and exclude the pages. you can also apply the trimming margin and aspect ratio.
Install Krop on ubuntu
Go to the krop download page and download the latest .deb file. Download size is very small, just 19KB. Double click the downloaded file. It will open the Ubuntu software. Just click the install button and type your ubuntu password. You can also install it via GDebi package manager.
After the successful installation restart ubuntu and click the Dash home from unity launcher. And type krop in the search field.Then click krop to open the installed krop app.