The short version
All three are capable PDF services. Smallpdf and iLovePDF offer broad online toolsets with usable free tiers that prompt an upgrade for heavier use; Adobe Acrobat is the professional standard and is paid after any trial. Which is 'best' depends on how often you work with PDFs and how much privacy and cost matter to you.
Smallpdf
A polished, widely used online suite covering most common tasks. The free tier works for occasional use but limits how much you can do before suggesting a paid plan. Good when you want a clean interface and do not mind the limits.
iLovePDF
A broad set of online PDF tools with free and paid tiers. The free tier covers basic tasks; heavier or batch workloads push toward a paid plan. A solid all-rounder similar in spirit to Smallpdf.
Adobe Acrobat
The industry standard for editing, reviewing, and managing PDFs, with the deepest feature set. It is a paid product — regular editing requires a subscription after any trial — so it suits professionals who work in PDFs daily.
Where a free in-browser tool fits
For one-off tasks — merge, split, compress, convert, sign — a free in-browser tool can be the better choice: no watermark, no sign-up, and your files stay on your device because nothing is uploaded. It will not replace Acrobat for heavy professional editing, but for everyday jobs it removes the trial walls entirely.
How to decide
Daily professional PDF work, advanced editing, compliance features → Adobe Acrobat. Frequent mixed tasks with a tidy UI → Smallpdf or iLovePDF (mind the free-tier limits). Occasional tasks where free and private matter most → a free in-browser tool. Pricing and limits change often, so confirm the current terms before committing.