There are quite a few websites that can handle documents and convert to PDFs. However, while there are many open source PDF converter applications, I have not seen an open source PDF converter web sites. Here are a few of the free sites I've come across:
PDF Converter - per the site:
Convert Word, Excel, PowerPoint, Images, Web Pages and other files to PDF
Primo PDF Online - per the site:
PrimoPDF's free Web-based PDF converter, PrimoOnline, is the simple
and fast way to create PDF files, online, without installing a thing.
Just upload your file or enter a URL, provide your email address, and
our server-based PDF creator will quickly convert it, and deliver the
PDF straight to your inbox.
PDF Online - Word to PDF only
DocuPub - per the site:
Convert many of the file formats (doc, docx, xls, ppt..) that are used daily to PDF or Image.
Adobe CreatePDF - per the site:
Just upload a file and create a PDF document with one click... [and] Upload two or more files. Get a single PDF file with the documents in the order you want to show them.