The PDF cannot display the full content of a single line of text.

[allrobot]

If I select and copy, I can see that this is the complete text "年平均工资" but it is truncated and only displays "年平".

统计.pdf

[GitHubRulesOK]

Cannot comment accurately without the specific file but that problem is not uncommon with PDF, where there are multiple facets to text the raw data is binary 01000101 (often compressed) which is decoded into more visible characters like Latin ASCII or WinAnsi like "þoÐ→" or text octets or doublebyte codes, which are then mapped to glyphs 年 平 均 工 using different spacing methodologies. so often words are one letter or a string of letters (as numbers) with non breaking spaces or no spaces the problem is that last stage where you select the decoded binary but the glyph is not the correct format to be extracted in which case it may convert to nulls or just the white spaces.

First always test in latest pre-release https://www.sumatrapdfreader.org/prerelease as some decodings or alternate recodings may have improved over time.
If still a problem in latest pre-release a copy of the file is need to analyse and see if its a fault in PDF or in decoding.

[allrobot]

https://github.com/sumatrapdfreader/sumatrapdf/files/15262180/default.pdf
I used sumatrapdf is latest...

[GitHubRulesOK]

Thanks for the sample it is a faulty line definition in the PDF as we can see here in Adobe Acrobat Reader

Oddly the file says it was written using Adobe Software so perhaps an inexperienced user or no checker. It can be fixed in an Editor.

[allrobot]

You mean maybe the author didn't check for this error when converting from Word to PDF?(Since it is unrelated to the PDF reader, this error will not be fixed?)

[GitHubRulesOK]

SumatraPDF cannot correct what Adobe cannot get right.
It is the Author made some mistake even if that mistake was using Adobe Design Software.

The person responsible for Quality Assurance Checking the PDF generated either was on sick leave or did not look at the PDF quality for flaws in that area, and ask it be regenerated correctly.

Computer fails are more spectacular than Human ones and I dont have time to check how many other fails are in the file.
If it is important then complain to the file Author.