Finding a single piece of software that handles image compression, merging, and digital signing can be a challenge, as these features usually live in different "neighborhoods" of the software world. However, the rise of all-in-one productivity suites has made this easier. The All-in-One Powerhouse: PDF Suites

To efficiently manage images—whether you need to compress them for web use, merge multiple files, or add digital signatures—there are several specialized and all-in-one software solutions available for download across Windows, macOS, and mobile platforms. All-in-One Solutions (Compress, Merge, & Sign)

ILovePDF (Desktop Version): Known for its simplicity, this tool offers a dedicated "Compress, Merge, and Sign" workflow that is incredibly user-friendly.

Adobe Acrobat Pro: The gold standard for merging and signing, with powerful built-in compression.