Every year, millions of UK taxpayers file a Self Assessment tax return with HM Revenue and Customs (HMRC). Whether you are self-employed, a landlord, a company director or earning above the £100,000 threshold, the process involves gathering forms, filling them accurately and submitting them before the deadline. In this guide, we show you how to use free, browser-based PDF tools to prepare your HMRC Self Assessment — from filling the SA100 to merging supporting documents and compressing the final packet — all without expensive software.
What Is HMRC Self Assessment?
Self Assessment is the system HMRC uses to collect Income Tax. Tax is usually deducted automatically from wages, pensions and savings (via PAYE), but if you are self-employed, a partner in a partnership, or have income above certain thresholds, you must report your income and capital gains and claim tax reliefs by filing a tax return.
The main form is the SA100. Depending on your circumstances, you may also need supplementary pages: SA102 (employment), SA103 (self-employment), SA105 (UK property), SA106 (foreign income), or SA108 (capital gains). HMRC publishes all of these as fillable PDFs on their website.
HMRC PDF Forms Explained
HMRC provides downloadable PDF forms at gov.uk/self-assessment-tax-return-forms. These are fillable PDFs — you can open them in any modern browser, click on each field and type the required information. The forms include interactive checkboxes, dropdown menus and calculation fields for common totals.
You do not need Adobe Acrobat or any paid software to fill these forms. Your browser's built-in PDF viewer (Chrome, Edge, Safari, Firefox) handles form filling natively. Once filled, save the form to your device using Ctrl+S or Cmd+S.
Filling SA100 and Supplementary Pages
Download the SA100 from the GOV.UK website and open it in your browser. Fill in your personal details (name, UTR number, National Insurance number, address), then complete the relevant income sections. If you are self-employed, download and fill SA103 as well. If you have rental income, fill SA105.
Tips for accurate filing: double-check your Unique Taxpayer Reference (UTR) number — a single wrong digit will delay processing. Use your P60 or P45 for employment income figures. Keep digital copies of all receipts and invoices for at least 22 months after the tax year ends, as HMRC may request them.
Merging Supporting Documents
A complete Self Assessment often includes multiple documents: the SA100, supplementary pages, P60, P11D (benefits), dividend vouchers and receipts. Keeping these as separate files is messy and increases the risk of losing a page. Merging them into a single PDF creates a clean, organized packet.
Using the merge function in Automarkly PDF Tools, combine all your documents in the correct order. Name files with a numeric prefix (01-SA100.pdf, 02-SA103.pdf, 03-P60.pdf) before merging to ensure they appear in the right sequence. This is especially useful if you are sending the packet to an accountant or storing it for your records.
Compressing and Submitting
Scanned documents (P60s, receipts, invoices) can be large. A complete tax packet with multiple scanned pages can easily exceed 10-15 MB. If you need to email the packet to your accountant or upload it to a portal, compression is essential.
Automarkly's PDF compression tool reduces file size by 50-70% while keeping text crisp. Set the quality slider to 75-80% for tax documents — this preserves the legibility of numbers and text while dramatically reducing file size. Always preview the compressed file to ensure all figures are readable before submitting.
Keeping Your Data Secure
Tax documents contain your National Insurance number, UTR, income details and bank information. This is a goldmine for identity thieves. Never upload tax documents to an unverified online tool that processes files on a remote server. Always use a client-side tool like Automarkly, where all processing happens in your browser's memory.
Additional UK-specific security tips: file through the official HMRC online portal, not via email. Be alert to phishing — HMRC will never ask for your bank details by email or text. Store your final tax packet in an encrypted folder. Delete temporary copies from your Downloads folder after merging. If you use an accountant, use a secure portal rather than standard email to transfer documents.
Filing your UK Self Assessment does not require paid software. With Automarkly's free PDF Tools, you can fill, merge, compress and manage your HMRC tax forms entirely in your browser — keeping your sensitive financial data private and secure. Try it for your next tax return.