Most file converter tools upload your files to a server. That's fine for non-sensitive content, but for anything personal or confidential, it's worth knowing what actually happens to your data. Here's an honest comparison — including which tools process files locally so nothing ever gets uploaded.
Quick answer: For private documents (tax returns, contracts, medical files), use a browser-based tool that processes locally. For PDF-heavy work without privacy concerns, Smallpdf or ILovePDF are solid. For unusual or rare format conversions, CloudConvert has the broadest coverage.
What should you look for in a file converter?
Before comparing tools, here's what actually matters:
Privacy — Does it upload your files to a server, or process locally? For personal and work documents, this matters.
Format support — Does it handle the specific conversion you need?
Output quality — Does the output look good, or does quality degrade significantly?
File size limits — Free tiers often have limits. Are they high enough for your use case?
Speed — How long does it take?
No dark patterns — Does it try to trick you into signing up, paying, or downloading something you didn't intend?
Which tools process your files locally without uploading?
These tools run in your browser. Your file never leaves your device. You can browse all available tools to find exactly what you need.
What they're good at:
- JPG/PNG/WebP image conversion
- PDF manipulation (merge, split, compress)
- Audio conversion (MP3, WAV, M4A)
- Video conversion (MP4, WebM, MKV)
- Text tools (JSON formatting, Base64 encoding, etc.)
Limitations:
- Very large files (1GB+ videos) may be slow due to browser memory limits
- First-time use of FFmpeg-based tools requires a ~30MB download (cached afterward)
Best for: Everyday conversions, sensitive documents, any file you don't want uploaded.
How does Smallpdf compare?
Well-designed tool focused on PDFs. Clean interface, good results for PDF-specific operations.
Strong: PDF compression, merge, split, convert from Office formats, e-signing Weak: Limited to PDFs and basic Office-to-PDF conversion Privacy: Uploads to their servers. Says it deletes files after 1 hour. Limits: Free tier allows 2 tasks per day. Paid subscription ($12/month) removes limits. Verdict: Good for PDF work when you're OK with the upload and the daily limit isn't a problem. For PDF compression specifically, a browser-based PDF compressor avoids the upload entirely.
How does ILovePDF compare?
Similar to Smallpdf in scope — primarily PDF tools.
Strong: PDF tools, good compression quality, supports more operations than Smallpdf Weak: PDF-only focus Privacy: Uploads to servers. Deletes after processing. Limits: Free tier has usage limits and file size caps. Verdict: Good free PDF tool. Similar to Smallpdf; slightly more generous free tier.
How does CloudConvert compare?
Supports an enormous range of formats — 200+. True generalist converter.
Strong: Format breadth (audio, video, images, documents, archives — almost anything), API for automation Weak: Expensive for heavy use; costs add up quickly with the credit system Privacy: Uploads to their servers. Limits: 25 free conversions per day. Paid plans by conversion minutes or monthly. Verdict: Best option when you need a rare format conversion. Not cost-effective for frequent use.
What about Zamzar?
Another generalist converter with broad format support. Been around for a long time.
Strong: Very broad format support, email delivery of converted files Weak: Slower than newer services, dated interface Privacy: Uploads to their servers. Deletes after 24 hours. Limits: 50MB file size on free tier. Verdict: Solid for rare conversions. Not the first choice for common formats.
A friend once used one of these upload-based tools to convert a financial statement for a job application — didn't think twice about it. When I asked if they'd read the privacy policy, they hadn't. For anything work-related or personal, it's worth spending 30 seconds to check whether the tool uploads files.
What about Squoosh for images?
Specifically for image compression and format conversion. Runs entirely in your browser.
Strong: Excellent visual quality comparison tool, supports WebP/AVIF/MozJPEG, truly browser-based Weak: Single images only, no batch processing Privacy: 100% browser-based — no upload. Limits: None. Verdict: Best tool specifically for optimizing a single image for web use.
When should I use each tool?
| Situation | Tool to Use |
|---|---|
| Sensitive documents (anything private) | Browser-based local tool |
| PDF-specific work, share-worthy output | Smallpdf or ILovePDF |
| Unusual format conversion | CloudConvert or Zamzar |
| Image optimization for web | Squoosh |
| Batch image conversion | Browser-based bulk converter |
| Audio/video conversion | Browser-based (FFmpeg) |
What are the red flags to watch for?
"Install our extension to unlock" — You don't need an extension. Move on.
Aggressive upsell after one free use — Normal to some extent, but if you can't complete a basic task without paying, the tool isn't actually free.
Auto-download of software — Legitimate converter tools don't need to install anything.
No information about what happens to your files — If there's no privacy policy or explanation of file handling, be cautious.
Generic "converts everything" apps on mobile — Many mobile converter apps upload your files to servers and monetize them. Read reviews and the privacy policy.
What's the bottom line recommendation?
For most everyday conversions, a browser-based tool is the right default:
- No file uploads, full privacy
- No account needed
- No usage limits on common conversions
- Works for the most common format needs (PDF, images, audio, video)
For PDF-specific heavy use, Smallpdf or ILovePDF offer clean interfaces and good output quality. For unusual or rare formats, CloudConvert has the broadest coverage.
The tool you use should match the sensitivity of your files. For personal documents, tax records, or client files, browser-based is the only reasonable choice. For a photo of your vacation, any tool will do.
Frequently asked questions
Can I use these tools on my phone? Yes — browser-based tools work on mobile Chrome and Safari. The conversion speed depends on your phone's processor. Upload-based tools like Smallpdf work fine on mobile with a stable internet connection.
Is this completely free? Yes — no account, no payment, no watermark needed. You can use it as many times as you want.
Do my files get uploaded to a server? Not with browser-based tools. Everything runs directly in your browser using WebAssembly. Upload-based tools (Smallpdf, CloudConvert, etc.) do send your files to their servers.