I'm just some random new user, but -- I think it's possible to get a PDF, even now!
Diarium already lets you export to Word Documents and Web Pages, both which can be easily turned into PDFs.
Word Documents can be exported into PDFs using most common word processors.
And web browsers (like Chrome, for example) can print pages into PDFs. The exact steps vary depending on your OS, but the gist of it is:
1.) Export your diary pages into a Web Page.
2.) Open that web page in Chrome.
3.) In the Chrome toolbar, select the option to "Print.."
4.) Instead of selecting a printer, select the option that says "Save as PDF".
I hope this helps!