iOS (iPhone/iPad) — Apple's built-in PDF viewer (used in Safari, Mail, and Files) renders PDFs using the system's PDFKit framework. It supports basic text selection, search, annotations (markup tools in iOS 14+), and AcroForm filling. It does not support PDF JavaScript. Files app can open and annotate PDFs natively since iOS 11. Android — Android has no system-wide built-in PDF viewer. Chrome on Android uses PDFium (the same engine as Chrome on desktop) to display PDFs opened from the web. Gmail and Google Drive use a Google-built viewer. Third-party apps are needed for editing and signing. Best dedicated PDF apps for mobile: - Adobe Acrobat Reader (iOS/Android) — Most capable free PDF viewer on mobile. Supports AcroForm filling, signature tools, cloud storage sync, and annotation. Free with Adobe account; some features require paid subscription. - Foxit PDF Reader Mobile — Strong AcroForm support, annotation tools, and ConnectedPDF features. - PDF Expert (iOS only) — Well-reviewed iOS-first PDF editor with fast rendering, annotation, form filling, and merging. - Xodo (iOS/Android) — Free, cross-platform, good annotation and form filling, supports collaborative editing.

Works well on mobile: - Viewing and scrolling through standard PDF documents - Basic text search within a PDF - Filling AcroForm text fields, checkboxes, and dropdowns - Placing signature images (draw with finger or stylus, type, or upload image) - Basic annotations: highlight, underline, sticky notes - Sharing PDFs via email, AirDrop (iOS), or file sharing apps Does not work or works poorly: - XFA forms — the same viewer incompatibility that affects desktop browsers. XFA forms require Adobe Acrobat Reader on mobile and may still not render correctly on all mobile OS versions. - PDF JavaScript — mobile PDF viewers do not execute PDF JavaScript. Form calculations and validation scripts are not supported. - Complex annotation workflows — selecting, moving, and editing individual annotations on a small touch screen is difficult. Tablets (iPad with Apple Pencil) are significantly better for annotation than phones. - Large PDF files — PDFs over 20–30 MB may render slowly or crash mobile viewers. Compress large PDFs before sending to mobile users. - PDF Portfolios — mobile viewers do not display the portfolio Navigator interface; they show only the cover page or fail to show embedded files. Tips for PDF creators targeting mobile audiences: - Keep file sizes under 5 MB when possible — use a PDF compressor before distributing - Use large text (minimum 12pt) — small text is hard to read on phone screens without zooming - Use AcroForms with large tap targets for form fields — fields smaller than 44×44px are hard to tap - Test your PDF in Adobe Acrobat Reader on a phone before wide distribution

Electronic signing on mobile has become one of the primary use cases for mobile PDF apps. The main approaches: Draw with finger or stylus — Most PDF apps provide a signature pad where you draw your signature with a finger or Apple Pencil. The drawn signature is embedded as an image overlay. On an iPad with an Apple Pencil, this produces results close to a real handwritten signature. On a phone with a finger, the result is typically rougher. HowToGeek's mobile signing guide covers the main options per platform. Type a signature — Type your name and the app renders it in a handwriting-style font. Legally equivalent to a drawn image signature for E-Sign Act purposes. Upload a signature image — Photograph a handwritten signature on paper and upload it as a PNG with transparent background. Reusable across documents. Platform signing apps — DocuSign, Adobe Sign, and Dropbox Sign all have iOS and Android apps with full electronic signing workflows including audit trails, completion notifications, and secure storage. These are the appropriate tools for legally binding business contracts. For cryptographic digital signatures — These require a signing certificate stored on the device or on a cloud-based QTSP (Qualified Trust Service Provider). Adobe Acrobat Reader on mobile supports certificate-based digital signatures when the certificate is accessible; this is primarily an enterprise use case.

If you are creating PDFs that will primarily be viewed on mobile devices — marketing materials, product sheets, forms sent via SMS or WhatsApp — optimise for the mobile context: Reflow-aware layout — Single-column layouts work better on phone screens than multi-column. Content that reflows (tagged PDFs with correct reading order) can adapt to narrow screens in viewers that support PDF reflow (Adobe Reader's "Liquid mode" on mobile). Linearised PDFs — Optimised for fast web viewing; the first page loads before the rest of the file is downloaded. Use Acrobat's "Save as Optimized PDF" to create a linearised file. This significantly improves the experience of opening a large PDF over a mobile connection. Compressed images — Mobile connections (especially 3G/4G in regions with poor coverage) are slower than desktop broadband. Compress all embedded images. Use JPEG for photos (quality 70–80% is usually imperceptible) and PNG for diagrams. A PDF compressor that downsamples images to 150 DPI is appropriate for screen-only distribution. Accessible font sizes — Body text smaller than 11pt requires pinch-zoom on a phone. Use 12–14pt for primary content. Avoid text in images (cannot be zoomed without quality loss).