Convert PDF to PDF/A-1b using Ghostscript and check using veraPDF.

Makefile

# PDF/A-2b (PDF 1.7) converter using Ghostscript.
#
# Annotations must already be marked printable, or Ghostscript drops them. Use
#
#   \PassOptionsToPackage{pdfa}{hyperref}
#
# or similar in LaTeX.
#
#   make IN=input.pdf OUT=output.pdf convert
#   make FILE=file.pdf check
#   make files

GS      ?= gs
VERAPDF ?= verapdf
CURL    ?= curl
override GS      := $(shell command -v $(GS))
override VERAPDF := $(shell command -v $(VERAPDF))
override CURL    := $(shell command -v $(CURL))

DPI ?= 200

TMPDIR   ?= /tmp
WORKDIR  := $(patsubst %/,%,$(TMPDIR))/pdfa-$(shell id -u)
FILESDIR := $(WORKDIR)/files
BUILDDIR := $(WORKDIR)/build
ICC      := $(FILESDIR)/srgb.icc
TEMPLATE := $(FILESDIR)/PDFA_def.ps
PREFIX   := $(BUILDDIR)/PDFA_def.ps
BASEURL  := https://raw.githubusercontent.com/ArtifexSoftware/ghostpdl/ghostpdl-10.07.1

.DELETE_ON_ERROR:
.SUFFIXES:
MAKEFLAGS += --no-builtin-rules
.PHONY: convert check files

ifneq (,$(filter convert,$(MAKECMDGOALS)))
  ifeq (,$(strip $(IN)))
    $(error convert requires IN=<input.pdf>)
  endif
  ifeq (,$(strip $(OUT)))
    $(error convert requires OUT=<output.pdf>)
  endif
  ifeq (,$(GS))
    $(error Ghostscript not found -- install gs or pass GS=/path/to/gs)
  endif
  ifeq (,$(and $(wildcard $(ICC)),$(wildcard $(TEMPLATE))))
    ifeq (,$(CURL))
      $(error data files missing and curl not found -- install curl, or copy srgb.icc and PDFA_def.ps into $(FILESDIR))
    endif
  endif
endif
ifneq (,$(filter check,$(MAKECMDGOALS)))
  ifeq (,$(strip $(FILE)))
    $(error check requires FILE=<file.pdf>)
  endif
  ifeq (,$(VERAPDF))
    $(error veraPDF not found -- install verapdf or pass VERAPDF=/path/to/verapdf)
  endif
endif
ifneq (,$(filter files,$(MAKECMDGOALS)))
  ifeq (,$(CURL))
    $(error curl not found -- install curl or pass CURL=/path/to/curl)
  endif
endif

convert: $(OUT)

$(OUT): $(IN) $(PREFIX) $(ICC)
	$(GS) -dNOPAUSE -dBATCH -dQUIET -sDEVICE=pdfwrite -dCompatibilityLevel=1.7 \
		-dPDFA=2 -dPDFACompatibilityPolicy=1 -sColorConversionStrategy=RGB \
		-sProcessColorModel=DeviceRGB --permit-file-read=$(ICC) \
		-dDownsampleColorImages=true -dColorImageResolution=$(DPI) \
		-dColorImageFilter=/DCTEncode -dAutoFilterColorImages=false \
		-dDownsampleGrayImages=true -dGrayImageResolution=$(DPI) \
		-dGrayImageFilter=/DCTEncode -dAutoFilterGrayImages=false \
		-sOutputFile=$@ $(PREFIX) $<

$(PREFIX): $(TEMPLATE) $(MAKEFILE_LIST) | $(BUILDDIR)
	sed 's|^/ICCProfile (srgb.icc)|/ICCProfile ($(ICC))|' $< > $@

check: $(FILE)
	$(VERAPDF) -f 2b --nonpdfext $<

files: $(ICC) $(TEMPLATE)

$(ICC): | $(FILESDIR)
	$(CURL) -fsSL -o $@ $(BASEURL)/iccprofiles/srgb.icc

$(TEMPLATE): | $(FILESDIR)
	$(CURL) -fsSL -o $@ $(BASEURL)/lib/PDFA_def.ps

$(BUILDDIR) $(FILESDIR):
	mkdir -p $@