Scribus PDFs are quite heavy. And if you produce a web PDF with a standard Scribus export to Web options, you'll see that your file can take long to download because Scribus is mainly made to create print documents, for which the quality criteria will be very different. If you want the PDF files to be easily available on your website, it's particularly important to make them as light as possible to decrease download duration.
Reducing the size of your Scribus PDF will require the modification of some options here and there, especially in the PDF Export dialog. In fact, there is no absolute way to do it. It depends on the result that you want and the content of the file.
First of all, reduce the number of primary colors. If you need to print or display only in black, just use Grayscale PDF in the Color tab of the PDF Export dialog. If you want a web PDF, Screen/Web will be enough and avoid the use of the fourth primary and spot colors. Our postcard export to the Web instead...