CloudConvert can merge files of any supported file type to a single PDF file. For example, you could combine a stack of DOCX, JPG and PDF files to a single PDF file.
Therefore use the combine mode parameter, as the following example shows. Please note that the mode parameters needs to be set to combine in both the create process API request and the start process API request (refer to the documentation about the other necassary parameters for starting conversions).
Please have a look at the interactive merge PDF example, which shows how to implement this mode.