A JPG file is a single image file and a PDF file may have multiple pages. I don't understand why you want to convert multiple PDF files in to a single image file. But it is a normal process to convert multiple JPG images into a single PDF file. All you need is a PDF writer or a PDF printer driver.
Next- and perhaps most challenging’ pick the right tool for the job. If the most important part of your search is to find exactly what you’re looking for, it is likely that you’ll want to look at a tool for finding documents on the web. These include Google Docs, Evernote, and Microsoft Office Online (see our post on using Microsoft Office Online for image capture). And for documents that you may need to add-in as background for another project, a good alternative can be Office Lens. It uses Adobe Lightroom to help you organize your images on the web and the cloud more easily. It requires more setup and can’t do what you need for photo editing at this point, but it does a good job for organizing your images. If you use a Mac or PC, it can open in Photos.app on your Mac. Note that Lens can sometimes.