XLS is the pre-2007 Excel binary. PDF is a frozen page layout with no heading structure a machine can rely on. LibreOffice sits between the two, and this page says exactly what happens to an XLS file on the way to becoming a PDF one.
What runs when an XLS file becomes PDF
LibreOffice 24.2, on a machine we rent and watch. Your XLS file is uploaded once, LibreOffice runs once, the PDF comes back, and neither file is kept. XLS to PDF is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the XLS into the PDF
The XLS page layout, exactly. LibreOffice renders your XLS file the way it would print it, so margins, columns, page breaks, headers and footers land in the PDF where the author put them.
Fonts, embedded in the PDF, so your XLS document reads the same on a machine that has never had those fonts installed.
XLS tables, drawn as they were drawn. Borders, merged cells and column widths survive into the PDF because they are painted rather than re-described.
XLS cell formatting: number formats, colours and conditional fills print into the PDF as you see them on screen.
What XLS to PDF costs you
Editability. A PDF is the end of the line: nobody downstream can change a sentence without going back to your XLS file. Sometimes that is exactly the point, and sometimes it is exactly the problem.
Structure a machine can use. PDF has no heading hierarchy, no list semantics and no table model, so everything the XLS file knew about itself stops at the print stage. A program reading the PDF sees positioned glyphs and guesses the rest.
XLS formulas. Cells print into the PDF as their current values, the workbook logic does not travel, and neither does anything outside the XLS print range, which is what decides where the PDF pages cut.
Where XLS and PDF files come from
XLS. Excel wrote it before 2007, and things still produce it: old line-of-business software, machines nobody has updated, exports from systems written in the nineties. It stops at 65,536 rows and 256 columns, a ceiling XLSX raised sixteenfold, and a growing number of tools now refuse it outright because it is an OLE2 binary rather than a zip of XML.
If a model is going to read the PDF
Worth knowing before the PDF goes anywhere near an assistant: this is the wrong direction for that. A PDF is a set of positioned glyphs, and a model has to reconstruct the reading order before it can begin. If a machine rather than a person is going to read this XLS file, convert it to Markdown instead and skip the reconstruction entirely.
XLS to PDF, measured rather than promised
Running XLS to PDF against the real engine with a real file, LibreOffice wrote 18,519 bytes of PDF in 1.0 seconds, machine otherwise idle. That is one XLS file on one day and not an average, which is why the number is given together with the file that produced it.
The XLS to PDF verdict was reached by a format signature, meaning at least four bytes unique to the format were found at the right offset. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the PDF file has anything inside it.
The same XLS file, sent somewhere else
Other outputs the probe measured out of an XLS file, so the cost of choosing PDF can be read against something. Sizes do not compare across engines, because each family was probed with its own XLS witness file.
XLS to HTML: 2,101 bytes in 1.0 seconds, verified by reading the internal type and finding a witness word.
Other ways into PDF, and what they measured
Among the published routes into PDF, XLS is the number 16 largest output of the 26 measured. The witness files differ, so this ranks the probe run and not your document.
PPTX to PDF: 20,929 bytes in 1.2 seconds.
PPT to PDF: 20,740 bytes in 1.1 seconds.
ODT to PDF: 11,917 bytes in 1.1 seconds.
ODS to PDF: 12,884 bytes in 1.1 seconds.
ODP to PDF: 20,929 bytes in 1.3 seconds.
What we will not pretend about XLS to PDF
An XLS file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An XLS to PDF run that passes 90 seconds is killed, and the LibreOffice process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
LibreOffice is serialised on purpose, one document at a time per worker, because each soffice instance takes 150 to 300 MB on a machine with 3.8 GB. Measured ceiling: 23 XLS conversions in flight and roughly 0.9 a second sustained. Past that the queue answers 429 rather than falling over, which was tested at 32 at once.
One XLS file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
XLS to PDF: what people ask
What actually converts my XLS file to PDF?
LibreOffice does it, 24.2, on a machine we rent and watch. Not a browser trick and not somebody else service: the XLS file is uploaded once, LibreOffice runs once, the PDF comes back, and neither file is kept afterwards.
How long does XLS to PDF take?
On the file the probe used, LibreOffice took 1.0 seconds and wrote 18,519 bytes of PDF. That is one real measurement on one real XLS file, not an average and not a promise about yours: a larger XLS takes longer, and past 90 seconds the run is stopped.
What do I lose going from XLS to PDF?
The one to know about first: Editability. A PDF is the end of the line: nobody downstream can change a sentence without going back to your XLS file. Sometimes that is exactly the point, and sometimes it is exactly the problem.
Is the PDF searchable afterwards?
Yes. The text in your XLS file is written into the PDF as text, not as a picture of text, so it can be searched and copied. What the PDF does not have is structure: no headings, no list model, no table model. It reads well to a person and parses badly to a program.
Is XLS to PDF free?
There is a free allowance every month, and one XLS to PDF conversion costs half a credit against it. When the allowance runs out the tool says so and stops, rather than quietly handing you a worse PDF.