Only use SSL certificates for sections and services where they are truly needed. This is because encrypting all of the HTTP data stream requires a large number of CPU power. And so encrypting a page’s static elements such as images (except the content) uses up considerable resources while presenting no benefits.

Before putting them online, ensure that any PDFs are correctly optimized for the internet e.g. sampling rate and compression ratio of images, embedded fonts and resolution.
If not, allow the PDF to be downloaded chapter by chapter.
If you want to give users the ability to download a PDF reader, choose light software such as Sumatra (4.3 MB) instead of Adobe (48 MB), as this reduces bandwidth ten-fold for each download and, more importantly, uses less RAM (which helps fight against the digital divide and planned obsolescence).