Decide whether you are merging or splitting
Write down the outcome you actually want: several documents stitched into one to send out (merge), a few pages extracted from one file (split by range), or one file broken into chapters (split every page). The parameters differ completely — merging is about order, range splitting is about the interval, page splitting is about count and naming. Clarify the goal before opening a tool and you skip half the back-and-forth, especially the loop of "I split it, lost two pages, now I have to merge again".
Order is where merges go wrong
Most tools concatenate in the order you add files, while some sort by filename as well — mixing the two rules is the main source of shuffled pages. The reliable routine: rename files with two-digit prefixes (01_cover, 02_contract, 03_annex), then select them all at once. Whatever rule the tool applies, the result is correct. After merging, check two spots: whether page one is the cover, and whether the chapter boundary (between pages 10 and 11) lines up. Twenty seconds of checking prevents sending out a document in the wrong order.
Merging does not improve quality — but it does grow the file
Merging binds the pages together in order without re-compressing images, so quality matches the originals; total size is roughly the sum of the parts, sometimes a little larger because of metadata and the cross-reference table. The flip side: merging will not slim anything down. If the result exceeds an email or system limit, compress the PDFs individually first, then merge. Note also that most tools do not preserve bookmarks or form fields — if your PDFs contain fillable forms, check whether you still need to fill them after merging.
Three ways to split
Split by range: for extracting a chapter or a few pages of evidence; mind the difference between "3-7" (a continuous range) and "3,7" (two pages). Split every page: for archiving scans or running OCR; it produces many files, so use a common prefix to avoid overwriting. Split by count or size: for upload limits such as "5 MB per file". Splitting also does not re-encode content, but it does drop the original bookmark structure — keep a copy or export the outline if you need the table of contents.
Scans and password-protected files
PDFs with an open password or permission restrictions must be unlocked before merging or splitting, otherwise even the preview fails. Merging scans does not add a text layer, so searching inside the merged file will not work until OCR has run; the test is simple — try selecting text in the original, and if nothing highlights, it is image-only. OFD, the format common to Chinese e-invoices, cannot be handled by ordinary PDF tools: convert it to PDF or images in the browser first.
A pre-flight checklist
One, page count: the merged total should equal the sum of the parts (if not, a file failed to load). Two, orientation: with mixed portrait and landscape pages, readers may auto-rotate, so check that key pages are not upside down. Three, numbering: original page numbers repeat after merging, so add a continuous footer number for anything going out. Four, size: compress first if it exceeds the recipient’s limit. Five, privacy: contracts, ID cards and invoices should be cleaned up afterwards — do not leave intermediate copies on a shared computer.
想马上动手：用 PDF 工具箱 合并或拆分会直接给出页数与体积结果；如果合并后的文件需要提取文字，再用 PDF 转文字；遇到电子发票的 OFD 文件，可以先在 OFD 查看 里转成 PDF。Try it now: merge or split in the PDF toolbox and check page counts and size; extract text afterwards with PDF to text, or convert an OFD e-invoice to PDF in the OFD viewer.
FAQ
Does merging reduce quality?
No. Merging does not re-compress images and page content stays as it was; the file grows because every original is kept intact, not because the images changed.
Can password-protected PDFs be merged?
Unlock them with the original password first. These tools do not crack passwords: if a file cannot be opened, ask its owner for access.
Is there a size limit?
No server-side limit — processing happens on your device and is bounded by memory. Documents of tens of MB are easy; drawings of hundreds of MB are better merged in batches.