Multi PDF Merger is a free portable app to merge many files, documents, and images into a single PDF file. Many document types are supported, such as Microsoft Office, images, text files, and PDF and PostScript documents.

There are several online services for PDF merging; however, as far as we can tell, all of them are restricted in multiple ways (unless the user decides to pay for the service).

Supported applications and types include:

Microsoft Office:
Supported Office applications: Word, Excel, Powerpoint, Publisher, Visio. Additionally supported document types: text (*.txt), comma-separated values (*.csv), rich text format (*.rtf), OpenOffice/LibreOffice.

HTML documents and websites (URL):
A screenshot of the HTML page or the website is merged into the PDF document. The HTML document must be reasonably simple. Otherwise, a blank page is inserted instead.

Images:
Supported image file types: JPEG (*.jpg), GIF (*.gif), Portable Network Graphics (*.png), TIFF (*.tif), PostScript (*.ps, *.eps).