From the major Magazine Reader Experience Study research, undertaken for the American Association of Magazine Editors / Magazine Publishers of America…

“Key Findings: … Across magazines, page count has no impact on reader usage.”

On having a longer page count, in regard to the extra space available in PDF and other digital-only magazines, only 23% of readers strongly agree that…

“There is just too much magazine content available in [a magazine in] electronic form” (Magazine Media Readers and Tablets report, Nov 2011)

So the conclusion seems to be that adding a few extra pages to a PDF magazine, past a normal print magazines’s page count, won’t mean that reader usage is reduced because they feel overloaded with content.