I'm facing the same problem.
The Matrix I'm using has one static row and a dynamic row that ranges from 10 to say 15 columns. The report is in portrait format to accommodate the columns. Although the number of columns is high, all the columns are displayed on the same page. The problem is that each page is followed by a blank page when I export the report to .pdf. I haven't tried any other format because I need to export the data to specifically a pdf.
In design mode there is a large white space to the right of the matrix. A simple solution is to reduce the white space by pulling the margin closer to the matrix, but doing this is disturbing the look of the report. The report has a header which just doesn't sit well with the rest of the report when I drag the border down.
I guess I'm asking too much here but what I want is for SSRS to ignore the white spaces to the right of the matrix. Any ideas how I can get about doing this.