Prefix 01-, 02- so sorting matches the required sequence.
Only the scanned pages; leave text PDFs untouched.
Read the true page ranges off the merged file.
Tender documents almost always list required materials in a fixed sequence: business licence, tax registration, qualification certificates, past performance contracts, audited reports. Reviewers tick them off against that list, and a complete file in a different order reads as careless. Prefix the source files 01-, 02-, 03- so that name sorting matches the notice, then merge in one pass.
Portals typically cap the total upload and sometimes each page as well. A forty-page scan at 300 dpi reaches 60 MB easily. Compress before merging rather than after: compressing the finished file hands the compressor one big scan and often pushes small text below legibility, while compressing each scanned document lets you keep certificates sharp and flatten only the bulky photo pages.
If the notice asks for a contents page with page numbers, merge first and then read the real ranges off the merged file before writing that page. Doing it the other way round, guessing page counts before merging, is the most common reason a submission is returned for correction.
Re-printing a text PDF through a virtual printer rasterises or at least re-embeds everything, can double the size, and drops the original bookmarks. Merging assembles existing pages without rewriting content, so text stays vector and quality is untouched.
Check three things in order: total size, page count, and the filename. Most rejections are size; the next most common is a filename containing Chinese characters or spaces. Rename it to plain ASCII, re-check the total, and upload again — the content itself is rarely the issue.
Unlock it first; encrypted pages cannot be assembled.
It removes duplicated fonts and resources, so it saves a little, but it is not compression.
Only if the portal shows an outline — one entry per material helps reviewers.