PDF brochure

If the document is not protected by security, you can use the TouchUp Object tool to select images (right-click > edit image...). If you have Photoshop set up as your image editor (Edit > Preferences > TouchUp), the image will load in Photoshop and you can edit it there. To edit text, use the TouchUp text tool in Acrobat. This approach is really less than ideal, but may be your only option.

Open the PDF in Illustrator - everything will be preserved aside from things like forms, video and javascript so you can edit it and re-save as a PDF.

Be careful using Illustrator as a general purpose PDF editor.

According to D-v: http://forums.adobe.com/thread/509192

We have to give those kind of caveats as there are many things inside a PDF which can't be translated into [u]any[/u] other application - for example in a PDF we can have layers which only appear at specific zoom ranges, form fields, scripts, video, etc.

Given a "basic" PDF with text, artwork and no interactivity, AI will process it just fine. You will of course need to have the fonts installed on your system in order to edit the text, but that's it. Admittedly, back in the old days AI wasn't as good at working with PDF as it is now. You're never going to get "the original file" back unless it was also created in AI, but you'll be able to work with text, vector and raster objects, etc.