Portable document files, or PDF files, are meant to be used as replicas of actual documents. They were really not meant to be used for files that have not yet been edited and finalized. It is still prefer to have the original documents corrected and, afterwards, to create a new PDF. However, if the latter is not possible, then, editing of the PDF files may be done as a last resort using the following steps.
If you wish to change some portions of the text, you would need to have specific tools such as Adobe Acrobat, Adobe Illustrator, or Flipx PDF Editor. Take note that PDF file texts are stored as single lines. The text does not flow and create continuous columns as what you see in word-processing applications. In Adobe Acrobat, you can edit minor changes using a tool called TouchUp Text. Any added or deleted text will not move the rest of the text in the column. This sometimes creates problems with the rest of the words in the text. That is why this tool should be used only as a last resort.
Adobe Illustrator has a capability for opening different types of PDF files. However, just like Adobe Acrobat, it can be tricky to use it for altering text. This program does not give you much options on the choice of fonts. It has its own set of automatic fonts, which may not be compatible to the font used on the PDF being modified. It also creates problems on text flow, even on those portions that were not edited.
Inflx PDF Editor provides satisfactory results with regards to editing text and the flow of text on the rest of the document.
You have to be aware, however, that there is an automatic function in PDF that stores a duplicate of the original document. For instance, changing the text of a tagged PDF document will only make changes on the visible document, but not on its hidden duplicate. As a consequence, the original unaltered document can still show up in search engines even after the text has been modified.
If you want to edit images in a PDF file, you can make use of Photoshop and Illustrator with some of the features of Adobe Acrobat. Acrobat gives you options for different types of touchups. You can select the image you want to edit, and which particular object editor you will be using. The edit options appear once you use the touchup object tool to select the image to be edited and you click on it. The image gets re-embedded in the file after you're done with editing the image. You can also use these features for replacing the image with another image. It is advisable that you retain the original resolution of the image you opened. Insert the new image and save it to your PDF.
You can also turn a colored PDF into black and white. The tools for doing this are incorporated in the print production options of Acrobat X Pro and in older versions of Acrobat Pro.