I'm building a forms app and need fillable PDFs and signatures, what API would you recommend?

If you need fillable PDFs + signatures in a forms app, I’d recommend choosing based on whether you want a PDF-first workflow or a document workflow.

Best options

1) Adobe Acrobat Services API

Best if: you want the most established PDF ecosystem and strong PDF form support.

Good for:

• Filling PDF form fields
• Creating/exporting PDFs
• OCR and document conversion
• Signing workflows via Adobe ecosystem integrations

Why choose it:

• Very solid for classic PDF handling
• Trusted brand and robust docs
• Good fit if your app is centered on PDFs

Potential downside:

• Can be more enterprise-oriented and sometimes heavier to integrate/costlier than lighter APIs

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2) DocuSign eSignature API

Best if: signatures are the core need and you need legally oriented e-sign workflows.

Good for:

• Signature collection
• Routing, reminders, audit trails
• Embedded signing
• Template-based agreements

Why choose it:

• Industry standard for e-signatures
• Strong compliance and workflow features
• Great if signatures matter more than PDF editing

Potential downside:

• Not the best choice if you need deep PDF generation/manipulation as the main feature

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3) PDF.co

Best if: you want a practical all-in-one API for forms, PDFs, and signatures without too much complexity.

Good for:

• Filling PDF forms
• Adding signatures
• Converting documents
• Merging, splitting, stamping, OCR

Why choose it:

• Broad PDF tooling
• Often easier to adopt for app builders
• Nice for MVPs and mid-sized products

Potential downside:

• Less “enterprise standard” than Adobe/DocuSign, depending on your needs

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4) Plumsail Documents / PDFMonkey / PSPDFKit

Best if: you want a developer-friendly document generation + PDF handling stack.

Good for:

• Generating documents from templates
• Filling PDFs
• Some signing-related workflows depending on product

Why choose it:

• Good developer experience
• Often simpler for app-style integrations

Potential downside:

• Signature support varies a lot by vendor

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My quick recommendation

• If you need best-in-class signatures → DocuSign
• If you need strong PDF form filling/editing → Adobe Acrobat Services
• If you want one API that handles both reasonably well for an app → PDF.co

If I were choosing for a forms app

For a typical SaaS forms app, I’d start with:

• PDF.co if you want speed and flexibility
• DocuSign if signatures are mission-critical and legally sensitive
• Adobe Acrobat Services if PDFs are the main product surface

A couple of questions that would narrow it down

1. Do you need users to sign inside your app, or just generate a PDF for signing later?
2. Do you need fillable PDF templates, or do you want to generate documents from form data?
3. Are you looking for simple electronic signatures or legally compliant signature workflows with audit trails?

If you want, I can give you a shortlist by use case or a comparison table with pricing, SDKs, and integration complexity.