This is a wierd problem, one could face in SSRS. The export to PDF or Word, appears to break the columns in larger reports with 10 or more columns.
The main reason being the report size specifications.
You can check your report properties to find the report size specifications. Check the screenshot below:
Check the Width and Height properties of the dialog that pops up (Right-click on the Report -> Report Properties)
Find the width of your report tablix (in inches). This will be a good start. Do not worry about height now. Set the Width to the width property in the above dialog, from your calculation. This is indeed a trial and error process, until you get all the columns in one single page. You will also need to adjust the Margins accordingly.
This should solve the issue of columns being spilled over to the next page, while taking export to PDF or Word.
New Problem that can come up:
In the first page of the exported report, you see only one row in the tablix.
Adjust the margins to a lower value, less that 0.5in. They can be real culprits in this case.
Hope this helps with your SSRS issue.