PDF/A is a version of PDF designed for long-term archiving of electronic documents. PDF/A compliance is required for reporting purposes, such as when depositing publications into the NSF Public Access Repository.
Adobe Acrobat (not Adobe Reader) can convert from PDF to PDF/A. In Adobe Acrobat Pro Version 2024.002.20964, the All tools sidebar contains the option Apply PDF standards. Choose Save as PDF/A.
On macOS, the Preview app can convert from PDF to PDF/A. Choose File → Export..., then check the box Create PDF/A before saving.
Using the terminal, GhostScript can output PDF/A files with the command
gs -dPDFA=1 -dNOOUTERSAVE -sProcessColorModel=DeviceRGB -sDEVICE=pdfwrite -o output.pdf -dPDFACompatibilityPolicy=1 input.pdf
Use the pdfx package:
\usepackage[a-1b]{pdfx}
Choose Save As, select PDF for Save as type, click Options..., check the PDF/A compliant box under PDF options and click OK.
To verify a PDF/A file, use the verification tool veraPDF.