Yes, Reporter was designed to do this, because otherwise the graphs would be un readable. PDF reports need to be printable, which is why you can never display more than 25 rows on a graph without it being un-readable.
This symptom is the same whether you export the PDF report ( Download it to your workstation), archive it as a PDF file on the reporter server, schedule and/or e-mail the Report as a PDF file. The common denominator here, being that the report is always being converted to a PDF format.
NOTE: For information about a similiar issue on the Dasboard, see KB3899
Ways to workaround this are being discussed for the next version of Reporter.