Generally, for larger sized PDF files, there may be a limitation on how many pages you can convert at one time.  For such documents, if may be necessary to split the conversion into parts, by selecting a page range.  A successful conversion will usually depend on a) the system that you are currently running the software on b) hard drive space and RAM and c) type of data (i.e. text, graphics) and amount of content on each page. 

If you have any further questions, please do not hesitate to contact us at techsupport@investintech.com