PDF editing is a very needed feature but not every app out there provides that functionality. Not only that, the ones that enable PDF editing are very costly. For instance, take the case of PDFPen – that costs you $60 for the full version. Adobe’s propriety can cause a huge dent in your pocket: about $200 for the editors. Although PDF editors are costly, not everyone wants to edit PDF documents per se. More or less, it’s only to annotate, highlight or certain other research-oriented purposes. Much of this can be achieved through a variety of software that’s less costly or free. We’ll take a look at the options available for Mac users. Mac’s built-in solution, Preview, has become quite powerful over the years. One of the most impressive additions is the ability to annotate, highlight, adding images in a PDF file. Preview is as good as any other PDF editor out there but it is not a full-fledged editor: you can’t edit the entire PDF file as such. You can only take to a certain level of editing where you either add, or highlight sections of the PDF file. Preview can also help you combine files. Skim is a popular, open-source and free PDF reader that doubles up as a research tool with it set of features. It’s pretty much the same as Preview on Mac but it works good for those quick annotations, comments and highlights. PDFEscape is not exactly a traditional solution but if you’re okay with online PDF editors, this could very well fit your need. PDFEscape is a free online editor that works with any modern browser: you upload the file and start editing. Now that we’ve talked about PDF editors for the Mac, it’s also a good thing to take a look at some popular options available to do the same on iOS devices. There aren’t many popular apps or services for this, but if you’re ready to shell out some money, there are apps to help you annotate and read PDF docs on iOS devices. Right on top of the list is Good Reader. Sells at $4.99 and brings pretty good PDF reading and editing capability. The app has got a separate version for the iPad. Good Reader is good enough but you can’t fill/edit forms within the PDF file using Good Reader. PDF Expert is costlier than Good Reader but there are good features like scanning capabilities that make PDF Expert a little more advanced than Good Reader. Source.