1) Upload a PDF file and go to edit screen
2) Select the 'Modify Existing Text' tool, and click text user wants to change.
3) Depends on concrete pdf file, one line/one work/one character or nothing will be selected, 
   if nothing is selected, it simply meannsyou can not modify it
   (for example, the text is rendered as image)
4) Type in the corrected text and tab out to confirm changes,the change is marked using yellow color.
5) Select Menu/File/Export or Export with Option to export PDF file. Open the exported pdf file 
   using local pdf reader(Acrobat Reader for example) to check the result. If user did not introduce 
   new character doing the modification (for example, user changed text 'hello world' to 'world hello'),
   then there would be a good chance user can get expected result. If user did introduce new character, 
   Touchpdf.com will try best to use existing encoding/decoding to modify text, the result might be 
   expected or not-expected. And if some character is missing or some gibberish show up(either because
   the new character can't be decoded or there is no correndonding embedded font to render the character),
   user can double click the yellow text, and change it to blank which will result in removal of the 
   target text, after that, select 'Text Annotation' to add new text in the same position,user needs 
   to adjust font size and color to mimic existing text. 
6) Modify Existing Text does not impact page layout.