Cumulative month excel 2013 report

Have a rather large (30 page) database that is created monthly on our unique financial system in PDF format. It has so many titles and subtitles listed that it is virtually impossible to export it to a more user-friendly application such as excel 2013, so I manually cut and paste the data to excel. No problem so far and since there are not that many values and since they are in so many illogical locations, it doesn’t seem worthwhile to try and develop a sophisticated application to extract it into excel.
However, once data is in excel, it is used to produce a series of monthly cumulative reports and charts which seems to fall apart. For example, the month ending April 2016, I will need the running total of the months Oct - Mar (Oct +Nov+Dec…etc) plus the total for the end of April on these reports. So in effect, all I need is a formula for the prior month’s total (in this case, Mar total) added to Apr value for the Apr cell. Think I’ve got that point right so far but additionally, I’d like to have a formula in the remaining months (May –Sep) that would display a blank cell until the input is required for that applicable month.