It's not going to shock anyone to have a 5 MB file as a PDF
If I see a PDF containing a page or so of text and it turns out to be several MB, I would become a little suspicious. But you're right that most people are not aware of the general size of things.
With PDF, it would not be hard to obscure the presence of only a minimum of legitimate content using many page-breaks to give the illusion of a long document, and filling those pages with space-hogs such as large headers, tables, and algorithmically generated graphics. The sparsity of genuine content would not be too surprising given that PDF was originally intended for printing rather than reading.
If I see a PDF containing a page or so of text and it turns out to be several MB, I would become a little suspicious. But you're right that most people are not aware of the general size of things.