ILovePDF launched in 2010 out of Barcelona and is one of the most-used PDF tool sites on the web, with around 200 million users, native apps for iOS, Android, macOS and Windows, a developer API, and a free tier that lets you use most tools with file size and daily limits.
meraSaaS is a smaller, narrower product. It bundles 20 utility tools across PDFs, photos, business documents, and calculators. The PDF tool set is smaller than ILovePDF's. The difference is shape: meraSaaS adds non-PDF tools, runs everything in the browser, and uses a single $7 monthly price (with 7 of the 20 tools free).
If you mostly need PDF tools and you are comfortable with files going through a server for processing, ILovePDF is probably a better fit. If you want browser-side processing, multiple tool categories, or a flat low price without annual commitments, read on.