PDF Eraser is a simple tool which can erase images and text from a PDF file, cut or rotate pages, then add new text or image content of your own (plus a large watermark in the free version, unfortunately).
This is not a full-strength editor. You can't click in a paragraph of text, delete some words, insert others, and watch the text reflow. Instead you must treat the page as an image, and drag an eraser tool over it, then place a new text box or image over whatever content is left.
PDF Erase could still be enough for simple and informal tweaks, though - if you just need a quick way to remove a particular graphic, or a line of text.
There's also an option to rotate some or all of your document pages by 90 degree increments.
A bundled but separate tool can delete entire pages from your PDF (just select them from a list and click Delete).
When you're finished, PDF Eraser can save the edited document in just a few clicks.
If you can't live with the watermark then a Pro version is available for $29.95. This also allows commercial use of the program.