The most significant factor determining PDF file size is generally the images it contains.
iStudio Publisher embeds images in document (.ispx) files and exports images to PDF files at their original resolution and compression level, thus ensuring no loss of image quality. However, this approach can result in producing larger sized PDF files than are actually required for good quality output.
To achieve good image quality at the output stage, for printed documents images should maintain a minimum resolution in the document of 300 DPI, whereas a minimum of 72 DPI is usually sufficient for a document that will only be viewed on a computer screen.
One approach to reducing PDF file size is to examine the Effective DPI of images in a document, identify those having a significantly higher Effective DPI than the required 300 or 72, and replace them with new copies of the images that have been reduced in size using a separate image editing app (Preview, for example). However, this can be inconvenient, especially if two PDF versions of the same document are required, one for printing and the other for emailing.
A more convenient method is to keep original images in a document file, export a PDF file with full resolution images from iStudio, and then reduce the file size of this PDF export file using a separate app.
PDFLight is an excellent free app for reducing PDF size, especially when the PDF will only be viewed electronically. PDFLight reduces all images to 144 DPI and changes all images to have a device RGB color profile. Although PDFLight doesn’t have any configuration settings, it produces small files with images that look sharp when viewed on a screen.
PDF Squeezer is another excellent app, available at low cost and with a free demo version. PDF Squeezer maintains the existing color profile of each image and provides options over the amount of image compression.
Depending on the exact nature of a PDF and the amount of size reduction required, either of these apps might work a bit better than the other, and it's often worth trying both and comparing the results. PDFLight will generally produce PDFs with slightly sharper images for a given file size reduction, but by changing the image color profiles some of the image colors may be changed noticeably.